为什么 g++ 不关心初始值设定项列表分配给 (const std：：string&) a (std：：string)？和其他怪异

Question

我在做作业时遇到了这个问题。当你有一个继承类a的类B，而B通过在B的构造函数中调用a的构造函数来初始化a中的变量时，应该在a的构造函数中初始化的变量保持未初始化状态。如果我们创建一个类A的对象，这种情况似乎不会发生，尽管看起来唯一的区别是继承和构造函数链。

这里有一个最小的例子：

#include <iostream>
#include <string>
class A {
public:
A(std::string s)
: s_(s) {}
~A(){}
void Print() const {
std::cout << s_ + "123";
}
private:
const std::string& s_;
};
class B : public A {
public:
B(std::string s)
: A(s) {}
};
int main()
{
//A a = A("123");
//a.Print();
B b = B("123");
b.Print();
}

在Print((中，您可以删除额外的字符串文字。通过这种方式，问题集中在未初始化的变量上。但根据valgrind的说法，有了额外的文字，一百万字节仍然可以访问。这看起来很奇怪。

打印中没有文字的Valgrind((：

g++ -c -g -std=c++17 -Wall -Wextra -pedantic main.cpp -o main.o
g++ main.o   -o main
valgrind ./main
==22551== Memcheck, a memory error detector
==22551== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==22551== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==22551== Command: ./main
==22551==
==22551== error calling PR_SET_PTRACER, vgdb might block
==22551== Conditional jump or move depends on uninitialised value(s)
==22551==    at 0x4F4FA9A: std::basic_ostream<char, std::char_traits<char> >& std::__ostream_insert<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*, long) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.25)
==22551==    by 0x109BAF: A::Print() const (main.cpp:15)
==22551==    by 0x1099EB: main (main.cpp:32)
==22551==
==22551== Conditional jump or move depends on uninitialised value(s)
==22551==    at 0x545C928: fwrite (iofwrite.c:35)
==22551==    by 0x4F4FB83: std::basic_ostream<char, std::char_traits<char> >& std::__ostream_insert<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*, long) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.25)
==22551==    by 0x109BAF: A::Print() const (main.cpp:15)
==22551==    by 0x1099EB: main (main.cpp:32)
==22551==
==22551== Conditional jump or move depends on uninitialised value(s)
==22551==    at 0x54689B4: _IO_file_xsputn@@GLIBC_2.2.5 (fileops.c:1226)
==22551==    by 0x545C9E6: fwrite (iofwrite.c:39)
==22551==    by 0x4F4FB83: std::basic_ostream<char, std::char_traits<char> >& std::__ostream_insert<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*, long) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.25)
==22551==    by 0x109BAF: A::Print() const (main.cpp:15)
==22551==    by 0x1099EB: main (main.cpp:32)
==22551==
==22551== Conditional jump or move depends on uninitialised value(s)
==22551==    at 0x5468A85: _IO_file_xsputn@@GLIBC_2.2.5 (fileops.c:1275)
==22551==    by 0x545C9E6: fwrite (iofwrite.c:39)
==22551==    by 0x4F4FB83: std::basic_ostream<char, std::char_traits<char> >& std::__ostream_insert<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*, long) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.25)
==22551==    by 0x109BAF: A::Print() const (main.cpp:15)
==22551==    by 0x1099EB: main (main.cpp:32)
==22551==
==22551== Conditional jump or move depends on uninitialised value(s)
==22551==    at 0x5468210: _IO_file_write@@GLIBC_2.2.5 (fileops.c:1198)
==22551==    by 0x5468B9E: new_do_write (fileops.c:457)
==22551==    by 0x5468B9E: _IO_file_xsputn@@GLIBC_2.2.5 (fileops.c:1277)
==22551==    by 0x545C9E6: fwrite (iofwrite.c:39)
==22551==    by 0x4F4FB83: std::basic_ostream<char, std::char_traits<char> >& std::__ostream_insert<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*, long) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.25)
==22551==    by 0x109BAF: A::Print() const (main.cpp:15)
==22551==    by 0x1099EB: main (main.cpp:32)
==22551==
==22551== Syscall param write(buf) contains uninitialised byte(s)
==22551==    at 0x54ED264: write (write.c:27)
==22551==    by 0x546822C: _IO_file_write@@GLIBC_2.2.5 (fileops.c:1203)
==22551==    by 0x5468B9E: new_do_write (fileops.c:457)
==22551==    by 0x5468B9E: _IO_file_xsputn@@GLIBC_2.2.5 (fileops.c:1277)
==22551==    by 0x545C9E6: fwrite (iofwrite.c:39)
==22551==    by 0x4F4FB83: std::basic_ostream<char, std::char_traits<char> >& std::__ostream_insert<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*, long) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.25)
==22551==    by 0x109BAF: A::Print() const (main.cpp:15)
==22551==    by 0x1099EB: main (main.cpp:32)
==22551==
==22551== Syscall param write(count) contains uninitialised byte(s)
==22551==    at 0x54ED264: write (write.c:27)
==22551==    by 0x546822C: _IO_file_write@@GLIBC_2.2.5 (fileops.c:1203)
==22551==    by 0x5468B9E: new_do_write (fileops.c:457)
==22551==    by 0x5468B9E: _IO_file_xsputn@@GLIBC_2.2.5 (fileops.c:1277)
==22551==    by 0x545C9E6: fwrite (iofwrite.c:39)
==22551==    by 0x4F4FB83: std::basic_ostream<char, std::char_traits<char> >& std::__ostream_insert<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*, long) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.25)
==22551==    by 0x109BAF: A::Print() const (main.cpp:15)
==22551==    by 0x1099EB: main (main.cpp:32)
==22551==
==22551== Syscall param write(buf) points to uninitialised byte(s)
==22551==    at 0x54ED264: write (write.c:27)
==22551==    by 0x546822C: _IO_file_write@@GLIBC_2.2.5 (fileops.c:1203)
==22551==    by 0x5468B9E: new_do_write (fileops.c:457)
==22551==    by 0x5468B9E: _IO_file_xsputn@@GLIBC_2.2.5 (fileops.c:1277)
==22551==    by 0x545C9E6: fwrite (iofwrite.c:39)
==22551==    by 0x4F4FB83: std::basic_ostream<char, std::char_traits<char> >& std::__ostream_insert<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*, long) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.25)
==22551==    by 0x109BAF: A::Print() const (main.cpp:15)
==22551==    by 0x1099EB: main (main.cpp:32)
==22551==  Address 0x1ffefff910 is on thread 1's stack
==22551==  in frame #5, created by A::Print() const (main.cpp:14)
==22551==
==22551== Conditional jump or move depends on uninitialised value(s)
==22551==    at 0x5468BB0: new_do_write (fileops.c:458)
==22551==    by 0x5468BB0: _IO_file_xsputn@@GLIBC_2.2.5 (fileops.c:1277)
==22551==    by 0x545C9E6: fwrite (iofwrite.c:39)
==22551==    by 0x4F4FB83: std::basic_ostream<char, std::char_traits<char> >& std::__ostream_insert<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*, long) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.25)
==22551==    by 0x109BAF: A::Print() const (main.cpp:15)
==22551==    by 0x1099EB: main (main.cpp:32)
==22551==
==22551== Conditional jump or move depends on uninitialised value(s)
==22551==    at 0x5468BEB: _IO_file_xsputn@@GLIBC_2.2.5 (fileops.c:1279)
==22551==    by 0x545C9E6: fwrite (iofwrite.c:39)
==22551==    by 0x4F4FB83: std::basic_ostream<char, std::char_traits<char> >& std::__ostream_insert<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*, long) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.25)
==22551==    by 0x109BAF: A::Print() const (main.cpp:15)
==22551==    by 0x1099EB: main (main.cpp:32)
==22551==
==22551== Conditional jump or move depends on uninitialised value(s)
==22551==    at 0x545CA65: fwrite (iofwrite.c:45)
==22551==    by 0x4F4FB83: std::basic_ostream<char, std::char_traits<char> >& std::__ostream_insert<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*, long) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.25)
==22551==    by 0x109BAF: A::Print() const (main.cpp:15)
==22551==    by 0x1099EB: main (main.cpp:32)
==22551==
==22551== Conditional jump or move depends on uninitialised value(s)
==22551==    at 0x545CA6A: fwrite (iofwrite.c:45)
==22551==    by 0x4F4FB83: std::basic_ostream<char, std::char_traits<char> >& std::__ostream_insert<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*, long) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.25)
==22551==    by 0x109BAF: A::Print() const (main.cpp:15)
==22551==    by 0x1099EB: main (main.cpp:32)
==22551==
==22551== Conditional jump or move depends on uninitialised value(s)
==22551==    at 0x4F4FB91: std::basic_ostream<char, std::char_traits<char> >& std::__ostream_insert<char, std::char_traits<char> >(std::basic_ostream<char, std::char_traits<char> >&, char const*, long) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.25)
==22551==    by 0x109BAF: A::Print() const (main.cpp:15)
==22551==    by 0x1099EB: main (main.cpp:32)
==22551==
==22551==
==22551== HEAP SUMMARY:
==22551==     in use at exit: 0 bytes in 0 blocks
==22551==   total heap usage: 2 allocs, 2 frees, 73,216 bytes allocated
==22551==
==22551== All heap blocks were freed -- no leaks are possible
==22551==
==22551== For counts of detected and suppressed errors, rerun with: -v
==22551== Use --track-origins=yes to see where uninitialised values come from
==22551== ERROR SUMMARY: 13 errors from 13 contexts (suppressed: 0 from 0)

Valgrind with literal in Print((：

(为了适应角色的限制，我不得不把中间部分剪掉，整件事就在这里：https://pastebin.com/UQmB0mXj)

g++ -c -g -std=c++17 -Wall -Wextra -pedantic main.cpp -o main.o
g++ main.o   -o main
valgrind ./main
==22561== Memcheck, a memory error detector
==22561== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==22561== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==22561== Command: ./main
==22561==
==22561== error calling PR_SET_PTRACER, vgdb might block
==22561== Conditional jump or move depends on uninitialised value(s)
==22561==    at 0x10AC50: void std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_construct<char*>(char*, char*, std::forward_iterator_tag) (basic_string.tcc:217)
==22561==    by 0x10A7DD: void std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_construct_aux<char*>(char*, char*, std::__false_type) (basic_string.h:236)
==22561==    by 0x10A472: void std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_construct<char*>(char*, char*) (basic_string.h:255)
==22561==    by 0x10A15B: std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) (basic_string.h:440)
==22561==    by 0x10A05E: std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > std::operator+<char, std::char_traits<char>, std::allocator<char> >(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, char const*) (basic_string.h:5928)
==22561==    by 0x109E66: A::Print() const (main.cpp:15)
==22561==    by 0x109C8B: main (main.cpp:32)
==22561==
==22561== Conditional jump or move depends on uninitialised value(s)
==22561==    at 0x10AD84: std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_create(unsigned long&, unsigned long) (basic_string.tcc:137)
==22561==    by 0x10AC69: void std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_construct<char*>(char*, char*, std::forward_iterator_tag) (basic_string.tcc:219)
==22561==    by 0x10A7DD: void std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_construct_aux<char*>(char*, char*, std::__false_type) (basic_string.h:236)
==22561==    by 0x10A472: void std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_construct<char*>(char*, char*) (basic_string.h:255)
==22561==    by 0x10A15B: std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) (basic_string.h:440)
==22561==    by 0x10A05E: std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > std::operator+<char, std::char_traits<char>, std::allocator<char> >(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, char const*) (basic_string.h:5928)
==22561==    by 0x109E66: A::Print() const (main.cpp:15)
==22561==    by 0x109C8B: main (main.cpp:32)
==22561==
...
had to cut here to fit into character limit
...
==22561==
==22561== Conditional jump or move depends on uninitialised value(s)
==22561==    at 0x10A9F0: std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_S_copy(char*, char const*, unsigned long) (basic_string.h:337)
==22561==    by 0x10AED7: std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_S_copy_chars(char*, char*, char*) (basic_string.h:382)
==22561==    by 0x10ACB0: void std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_construct<char*>(char*, char*, std::forward_iterator_tag) (basic_string.tcc:225)
==22561==    by 0x10A7DD: void std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_construct_aux<char*>(char*, char*, std::__false_type) (basic_string.h:236)
==22561==    by 0x10A472: void std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_construct<char*>(char*, char*) (basic_string.h:255)
==22561==    by 0x10A15B: std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) (basic_string.h:440)
==22561==    by 0x10A05E: std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > std::operator+<char, std::char_traits<char>, std::allocator<char> >(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, char const*) (basic_string.h:5928)
==22561==    by 0x109E66: A::Print() const (main.cpp:15)
==22561==    by 0x109C8B: main (main.cpp:32)
==22561==
==22561== Conditional jump or move depends on uninitialised value(s)
==22561==    at 0x109DE1: std::char_traits<char>::copy(char*, char const*, unsigned long) (char_traits.h:348)
==22561==    by 0x10AA1D: std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_S_copy(char*, char const*, unsigned long) (basic_string.h:340)
==22561==    by 0x10AED7: std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_S_copy_chars(char*, char*, char*) (basic_string.h:382)
==22561==    by 0x10ACB0: void std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_construct<char*>(char*, char*, std::forward_iterator_tag) (basic_string.tcc:225)
==22561==    by 0x10A7DD: void std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_construct_aux<char*>(char*, char*, std::__false_type) (basic_string.h:236)
==22561==    by 0x10A472: void std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_construct<char*>(char*, char*) (basic_string.h:255)
==22561==    by 0x10A15B: std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) (basic_string.h:440)
==22561==    by 0x10A05E: std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > std::operator+<char, std::char_traits<char>, std::allocator<char> >(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, char const*) (basic_string.h:5928)
==22561==    by 0x109E66: A::Print() const (main.cpp:15)
==22561==    by 0x109C8B: main (main.cpp:32)
==22561==
==22561== Conditional jump or move depends on uninitialised value(s)
==22561==    at 0x4C366E6: memmove (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==22561==    by 0x109E05: std::char_traits<char>::copy(char*, char const*, unsigned long) (char_traits.h:350)
==22561==    by 0x10AA1D: std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_S_copy(char*, char const*, unsigned long) (basic_string.h:340)
==22561==    by 0x10AED7: std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_S_copy_chars(char*, char*, char*) (basic_string.h:382)
==22561==    by 0x10ACB0: void std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_construct<char*>(char*, char*, std::forward_iterator_tag) (basic_string.tcc:225)
==22561==    by 0x10A7DD: void std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_construct_aux<char*>(char*, char*, std::__false_type) (basic_string.h:236)
==22561==    by 0x10A472: void std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_construct<char*>(char*, char*) (basic_string.h:255)
==22561==    by 0x10A15B: std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) (basic_string.h:440)
==22561==    by 0x10A05E: std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > std::operator+<char, std::char_traits<char>, std::allocator<char> >(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, char const*) (basic_string.h:5928)
==22561==    by 0x109E66: A::Print() const (main.cpp:15)
==22561==    by 0x109C8B: main (main.cpp:32)
==22561==
==22561== Conditional jump or move depends on uninitialised value(s)
==22561==    at 0x4C36711: memmove (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==22561==    by 0x109E05: std::char_traits<char>::copy(char*, char const*, unsigned long) (char_traits.h:350)
==22561==    by 0x10AA1D: std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_S_copy(char*, char const*, unsigned long) (basic_string.h:340)
==22561==    by 0x10AED7: std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_S_copy_chars(char*, char*, char*) (basic_string.h:382)
==22561==    by 0x10ACB0: void std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_construct<char*>(char*, char*, std::forward_iterator_tag) (basic_string.tcc:225)
==22561==    by 0x10A7DD: void std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_construct_aux<char*>(char*, char*, std::__false_type) (basic_string.h:236)
==22561==    by 0x10A472: void std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_construct<char*>(char*, char*) (basic_string.h:255)
==22561==    by 0x10A15B: std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) (basic_string.h:440)
==22561==    by 0x10A05E: std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > std::operator+<char, std::char_traits<char>, std::allocator<char> >(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, char const*) (basic_string.h:5928)
==22561==    by 0x109E66: A::Print() const (main.cpp:15)
==22561==    by 0x109C8B: main (main.cpp:32)
==22561==
==22561== Conditional jump or move depends on uninitialised value(s)
==22561==    at 0x4C367EE: memmove (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==22561==    by 0x109E05: std::char_traits<char>::copy(char*, char const*, unsigned long) (char_traits.h:350)
==22561==    by 0x10AA1D: std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_S_copy(char*, char const*, unsigned long) (basic_string.h:340)
==22561==    by 0x10AED7: std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_S_copy_chars(char*, char*, char*) (basic_string.h:382)
==22561==    by 0x10ACB0: void std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_construct<char*>(char*, char*, std::forward_iterator_tag) (basic_string.tcc:225)
==22561==    by 0x10A7DD: void std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_construct_aux<char*>(char*, char*, std::__false_type) (basic_string.h:236)
==22561==    by 0x10A472: void std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_construct<char*>(char*, char*) (basic_string.h:255)
==22561==    by 0x10A15B: std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) (basic_string.h:440)
==22561==    by 0x10A05E: std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > std::operator+<char, std::char_traits<char>, std::allocator<char> >(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, char const*) (basic_string.h:5928)
==22561==    by 0x109E66: A::Print() const (main.cpp:15)
==22561==    by 0x109C8B: main (main.cpp:32)
==22561==
==22561== Invalid read of size 8
==22561==    at 0x4C367EE: memmove (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==22561==    by 0x109E05: std::char_traits<char>::copy(char*, char const*, unsigned long) (char_traits.h:350)
==22561==    by 0x10AA1D: std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_S_copy(char*, char const*, unsigned long) (basic_string.h:340)
==22561==    by 0x10AED7: std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_S_copy_chars(char*, char*, char*) (basic_string.h:382)
==22561==    by 0x10ACB0: void std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_construct<char*>(char*, char*, std::forward_iterator_tag) (basic_string.tcc:225)
==22561==    by 0x10A7DD: void std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_construct_aux<char*>(char*, char*, std::__false_type) (basic_string.h:236)
==22561==    by 0x10A472: void std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_construct<char*>(char*, char*) (basic_string.h:255)
==22561==    by 0x10A15B: std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) (basic_string.h:440)
==22561==    by 0x10A05E: std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > std::operator+<char, std::char_traits<char>, std::allocator<char> >(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, char const*) (basic_string.h:5928)
==22561==    by 0x109E66: A::Print() const (main.cpp:15)
==22561==    by 0x109C8B: main (main.cpp:32)
==22561==  Address 0x1fff001000 is not stack'd, malloc'd or (recently) free'd
==22561==
==22561==
==22561== Process terminating with default action of signal 11 (SIGSEGV)
==22561==  Access not within mapped region at address 0x1FFF001000
==22561==    at 0x4C367EE: memmove (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==22561==    by 0x109E05: std::char_traits<char>::copy(char*, char const*, unsigned long) (char_traits.h:350)
==22561==    by 0x10AA1D: std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_S_copy(char*, char const*, unsigned long) (basic_string.h:340)
==22561==    by 0x10AED7: std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_S_copy_chars(char*, char*, char*) (basic_string.h:382)
==22561==    by 0x10ACB0: void std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_construct<char*>(char*, char*, std::forward_iterator_tag) (basic_string.tcc:225)
==22561==    by 0x10A7DD: void std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_construct_aux<char*>(char*, char*, std::__false_type) (basic_string.h:236)
==22561==    by 0x10A472: void std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_construct<char*>(char*, char*) (basic_string.h:255)
==22561==    by 0x10A15B: std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) (basic_string.h:440)
==22561==    by 0x10A05E: std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > std::operator+<char, std::char_traits<char>, std::allocator<char> >(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, char const*) (basic_string.h:5928)
==22561==    by 0x109E66: A::Print() const (main.cpp:15)
==22561==    by 0x109C8B: main (main.cpp:32)
==22561==  If you believe this happened as a result of a stack
==22561==  overflow in your program's main thread (unlikely but
==22561==  possible), you can try to increase the size of the
==22561==  main thread stack using the --main-stacksize= flag.
==22561==  The main thread stack size used in this run was 8388608.
==22561==
==22561== HEAP SUMMARY:
==22561==     in use at exit: 1,089,515 bytes in 1 blocks
==22561==   total heap usage: 2 allocs, 1 frees, 1,162,219 bytes allocated
==22561==
==22561== LEAK SUMMARY:
==22561==    definitely lost: 0 bytes in 0 blocks
==22561==    indirectly lost: 0 bytes in 0 blocks
==22561==      possibly lost: 0 bytes in 0 blocks
==22561==    still reachable: 1,089,515 bytes in 1 blocks
==22561==         suppressed: 0 bytes in 0 blocks
==22561== Rerun with --leak-check=full to see details of leaked memory
==22561==
==22561== For counts of detected and suppressed errors, rerun with: -v
==22561== Use --track-origins=yes to see where uninitialised values come from
==22561== ERROR SUMMARY: 197 errors from 12 contexts (suppressed: 0 from 0)
src.make:32: recipe for target 'valgrind-run' failed
make: *** [valgrind-run] Segmentation fault (core dumped)

我的问题是：

1. 为什么编译器不会因为A的构造函数而给出错误？我们如何设置具有该值的引用？

2. 为什么只有在涉及继承时才会出现错误？

3. 为什么在Print((中添加文字会产生如此大的效果？为什么会因此而分配一百万字节？

Answer 1

1. 为什么编译器不会因为A的构造函数而给出错误

这可能是因为A的构造函数格式良好，因此编译器必须接受它才能符合标准。

您可以将引用绑定到局部变量。在构造函数返回后，引用将无效，但如果程序在从构造函数返回后从不间接通过引用，那么从技术上讲这是没有问题的。编译器证明程序会做到这一点并非易事(这个问题通常类似于停顿问题(。

编译器确实通过以下方式发出警告：

警告：将引用成员"s_"绑定到堆栈分配的参数"s"[-Wdanglingfield]

---

我们如何使用该值设置引用？

您已将引用绑定到局部变量。目前还不清楚你想做什么，但可能你不应该使用参考成员来实现这一点。

---

2. 为什么

3. 为什么

因为程序的行为是未定义的。任何更改都可能以任何方式影响程序的行为。行为甚至可以在不改变程序的情况下改变。或者它可能不会改变。该程序的行为没有任何保证。