将 boost::时序机制包装成类编译错误?

Question

我遇到的问题与在 a.cpp 中写成t1_t0 = t1 - t0;的类似类型之间的操作结果类型的声明有关。t1和t0分别是最终时间和初始时间，差值用于测量一次计算之间的时间跨度。我尝试在整个函数中使用auto来代替变量boost::chrono::high_resolution_clock::time_point、std::chrono::seconds和int，但这在函数之前被调用后，在声明变量时引起了奇怪的问题。

它被称为error: use of before deduction of ‘auto’编译错误，所以我随后尝试使用真实类型编译项目，现在我遇到了这个麻烦。如果类在上面声明，项目确实可以很好地编译(使用autoint main)，但如果将类分为头文件和 cpp 文件，则项目不起作用。

这是整个项目，请帮忙！

#include <iostream>
#include "a.h"
int main()
{
a A;
int timer = A.time();
std::cout<<timer<<std::endl;
return 0;
}

这是A.H

#ifndef A_H
#define A_H
#include <iostream>
#include <boost/chrono/chrono_io.hpp>
#include <boost/date_time/posix_time/posix_time.hpp>
#include <boost/thread/thread.hpp>
#include <sstream>
#include <cassert>
#include <chrono>
class a
{
private:
public:
a();
~a();
int time();
};
#endif

这是 A.cpp

#include "a.h"
a::a(){}
a::~a(){}
int a::time()//simple benchmark timer
{
boost::chrono::high_resolution_clock::time_point t0, t1, t1_t0;
t0 = boost::chrono::high_resolution_clock::now();
//Run a process to measure time.
t1 = boost::chrono::high_resolution_clock::now();
t1_t0 = t1 - t0;//Not sure what the resulting type is here?
std::chrono::seconds nsec = std::chrono::duration_cast<std::chrono::seconds>(t1_t0);
//Not sure what the resulting type is here?
return nsec.count();//Not sure what the return type is here?
}

以下是编译它的错误。

||=== Build: Debug in return_type_auto_from_class (compiler: GNU GCC Compiler) ===|
/home/Desktop/a.cpp||In member function ‘int a::time()’:|
/home/Desktop/a.cpp|14|error: no match for ‘operator=’ (operand types are ‘boost::chrono::steady_clock::time_point {aka boost::chrono::time_point<boost::chrono::steady_clock>}’ and ‘boost::common_type<boost::chrono::duration<long int, boost::ratio<1l, 1000000000l> >, boost::chrono::duration<long int, boost::ratio<1l, 1000000000l> > >::type {aka boost::chrono::duration<long int, boost::ratio<1l, 1000000000l> >}’)|
/usr/include/boost/chrono/time_point.hpp|156|note: candidate: constexpr boost::chrono::time_point<boost::chrono::steady_clock>& boost::chrono::time_point<boost::chrono::steady_clock>::operator=(const boost::chrono::time_point<boost::chrono::steady_clock>&)|
/usr/include/boost/chrono/time_point.hpp|156|note:   no known conversion for argument 1 from ‘boost::common_type<boost::chrono::duration<long int, boost::ratio<1l, 1000000000l> >, boost::chrono::duration<long int, boost::ratio<1l, 1000000000l> > >::type {aka boost::chrono::duration<long int, boost::ratio<1l, 1000000000l> >}’ to ‘const boost::chrono::time_point<boost::chrono::steady_clock>&’|
/usr/include/boost/chrono/time_point.hpp|156|note: candidate: constexpr boost::chrono::time_point<boost::chrono::steady_clock>& boost::chrono::time_point<boost::chrono::steady_clock>::operator=(boost::chrono::time_point<boost::chrono::steady_clock>&&)|
/usr/include/boost/chrono/time_point.hpp|156|note:   no known conversion for argument 1 from ‘boost::common_type<boost::chrono::duration<long int, boost::ratio<1l, 1000000000l> >, boost::chrono::duration<long int, boost::ratio<1l, 1000000000l> > >::type {aka boost::chrono::duration<long int, boost::ratio<1l, 1000000000l> >}’ to ‘boost::chrono::time_point<boost::chrono::steady_clock>&&’|
/home/Desktop/a.cpp|16|error: no matching function for call to ‘duration_cast(boost::chrono::steady_clock::time_point&)’|
/usr/include/c++/5/chrono|194|note: candidate: template<class _ToDur, class _Rep, class _Period> constexpr typename std::enable_if<std::chrono::__is_duration<_Tp>::value, _ToDur>::type std::chrono::duration_cast(const std::chrono::duration<_Rep, _Period>&)|
/usr/include/c++/5/chrono|194|note:   template argument deduction/substitution failed:|
/home/Desktop/a.cpp|16|note:   ‘boost::chrono::steady_clock::time_point {aka boost::chrono::time_point<boost::chrono::steady_clock>}’ is not derived from ‘const std::chrono::duration<_Rep, _Period>’|
||=== Build failed: 2 error(s), 0 warning(s) (0 minute(s), 2 second(s)) ===|

编辑的代码，这确实修复了原始代码，现在可以工作了。

int a::time()
{
boost::chrono::high_resolution_clock::time_point t0, t1;//, t1_t0;
t0 = boost::chrono::high_resolution_clock::now();
//Run a process to measure time.
boost::this_thread::sleep_for( boost::chrono::milliseconds{ 2000});
t1 = boost::chrono::high_resolution_clock::now();
auto t1_t0 = t1 - t0;
auto nsec = boost::chrono::duration_cast<boost::chrono::seconds>(t1_t0);
return nsec.count();
}

Answer 1

1. 不要boost::chrono和std::chrono混为一谈。 使用其中之一。 我建议std::chrono如果您的平台提供和支持它，否则请使用boost::chrono。

2. 两个time_point之间的区别是一个duration，而不是另一个time_point。 例如，"明天"和"今天"可以被认为是时间点(精度非常粗略)。tomorrow - today == 1 day. 一天是一个持续时间。

3. 如果你想要纳秒的最终结果，没有必要那么努力：

   nanoseconds nsec = t1 - t0;

4. 返回nanoseconds而不是从a::time()int怎么样？ 这样，客户端就具有类型安全的持续时间，而不是可能意味着任何内容的int。

   return t1 - t0;