我有以下代码：
#include <cstdint>
#include <cstdlib>
#include <memory>
#include <string>
#include <sstream>
class CustomAllocator
{
public:
CustomAllocator(const std::size_t sizeBytes,
void* const start)
:
m_sizeBytes(sizeBytes),
m_usedBytes(0),
m_start(start),
m_current(start)
{
}
void* Allocate(const std::size_t& numBytes,
const std::uintptr_t& alignment)
{
std::size_t space = m_sizeBytes - m_usedBytes;
if(std::align(alignment, numBytes, m_current, space))
{
// the amount used for alignment
m_usedBytes += (m_sizeBytes-m_usedBytes) - space;
// the amount actually needed
m_usedBytes += numBytes;
void* address = m_current;
m_current = reinterpret_cast<void*>(
reinterpret_cast<std::uintptr_t>(m_current) + numBytes);
return address;
}
throw std::bad_alloc();
}
void Free(void* const ptr)
{
// do nothing in this Allocator, but other derived types may
}
void Clear()
{
m_current = m_start;
m_usedBytes = 0;
}
std::size_t GetSize() const { return m_sizeBytes; }
void* const GetStart() const { return m_start; }
protected:
const std::size_t m_sizeBytes;
std::size_t m_usedBytes;
void* const m_start;
void* m_current;
};
// many types derive from base CustomAllocator type
// allows for my custom allocators to be used in STL containers
template<typename T, typename Alloc>
class STLAdaptor
{
public:
typedef T value_type;
template<typename U>
struct rebind
{
typedef STLAdaptor<U, Alloc> other;
};
STLAdaptor(Alloc& allocator)
:
m_allocator(allocator)
{
}
template<typename U>
STLAdaptor(const STLAdaptor<U, Alloc>& other) noexcept
:
m_allocator(other.m_allocator)
{}
[[nodiscard]] constexpr T* allocate(std::size_t n)
{
return reinterpret_cast<T*>
(m_allocator.Allocate(n * sizeof(T), alignof(T)));
}
constexpr void deallocate(T* p, std::size_t n)
{
m_allocator.Free(p);
}
std::size_t MaxAllocationSize() const
{
return m_allocator.GetSize();
}
bool operator==(const STLAdaptor<T,Alloc>& rhs)
{
return m_allocator.GetStart() == rhs.m_allocator.GetStart();
}
bool operator!=(const STLAdaptor<T,Alloc>& rhs)
{
return !(*this == rhs);
}
Alloc& m_allocator;
};
int main()
{
const std::size_t memSize = 10000000;
void* mem = std::malloc(memSize);
CustomAllocator customAlloc(memSize, mem);
std::basic_string<char, std::char_traits<char>,
STLAdaptor<char, CustomAllocator>>
myString(customAlloc);
myString = "something something";
std::basic_istringstream<char, std::char_traits<char>,
STLAdaptor<char, CustomAllocator>>
iss(myString, std::ios_base::in, customAlloc);
std::free(mem);
return 0;
}
使用GCC 10.2-Wall -fexceptions -std=c++2a -g编译时，我得到以下错误：
||=== Build: Debug in CPPTESTS (compiler: Mingw-8.1.0 32bit) ===|
C:CPPTESTSmain.cpp||In function 'int main()':|
C:CPPTESTSmain.cpp|136|error: no matching function for call to 'std::__cxx11::basic_istringstream<char, std::char_traits<char>, STLAdaptor<char, CustomAllocator> >::basic_istringstream(std::__cxx11::basic_string<char, std::char_traits<char>, STLAdaptor<char, CustomAllocator> >&, const openmode&, CustomAllocator&)'|
C:MSYS2-32mingw32includec++10.2.0sstream|477|note: candidate: 'std::__cxx11::basic_istringstream<_CharT, _Traits, _Alloc>::basic_istringstream(std::__cxx11::basic_istringstream<_CharT, _Traits, _Alloc>&&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = STLAdaptor<char, CustomAllocator>]'|
C:MSYS2-32mingw32includec++10.2.0sstream|477|note: candidate expects 1 argument, 3 provided|
C:MSYS2-32mingw32includec++10.2.0sstream|460|note: candidate: 'std::__cxx11::basic_istringstream<_CharT, _Traits, _Alloc>::basic_istringstream(const __string_type&, std::ios_base::openmode) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = STLAdaptor<char, CustomAllocator>; std::__cxx11::basic_istringstream<_CharT, _Traits, _Alloc>::__string_type = std::__cxx11::basic_string<char, std::char_traits<char>, STLAdaptor<char, CustomAllocator> >; std::ios_base::openmode = std::ios_base::openmode]'|
C:MSYS2-32mingw32includec++10.2.0sstream|460|note: candidate expects 2 arguments, 3 provided|
C:MSYS2-32mingw32includec++10.2.0sstream|442|note: candidate: 'std::__cxx11::basic_istringstream<_CharT, _Traits, _Alloc>::basic_istringstream(std::ios_base::openmode) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = STLAdaptor<char, CustomAllocator>; std::ios_base::openmode = std::ios_base::openmode]'|
C:MSYS2-32mingw32includec++10.2.0sstream|442|note: candidate expects 1 argument, 3 provided|
C:MSYS2-32mingw32includec++10.2.0sstream|425|note: candidate: 'std::__cxx11::basic_istringstream<_CharT, _Traits, _Alloc>::basic_istringstream() [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = STLAdaptor<char, CustomAllocator>]'|
C:MSYS2-32mingw32includec++10.2.0sstream|425|note: candidate expects 0 arguments, 3 provided|
||=== Build failed: 1 error(s), 0 warning(s) (0 minute(s), 1 second(s)) ===|
查看<sstream>中std::basic_istringstream的代码，可以发现https://en.cppreference.com/w/cpp/io/basic_istringstream/basic_istringstream不存在，这当然解释了我的错误。
但如果是这样的话，我究竟该如何用有状态的自定义分配器来构造这样一个对象呢？
版本gcc 10.2似乎不支持std:：basic_istringstream的C++20构造函数。
在C++20正式标准化之前，您可以在10.2之后使用gcc trunk，它支持C++20构造函数，因此编译代码时不会出错：
活螺栓示例