c++ - 将 boost::chrono 计时机制包装到类编译错误中？

Question

我遇到的问题与在 a.cpp 中写为 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，其差值用于衡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 ，但是在声明变量之前调用函数之后，这会导致奇怪的问题。

它被称为 error: use of before deduction of ‘auto’ 编译错误，所以我尝试使用真实类型编译项目，但现在我遇到了这个麻烦。如果类在 int main 上方声明，该项目确实可以正常编译(使用 auto)，但如果该类被分为头文件和 cpp 文件，则该项目无法正常工作.

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;
}

这是啊

#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?
}

这是编译它的错误。

||=== 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)) ===|

编辑过的代码，这确实修复了原始代码，现在可以使用了。

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. 不要混合使用 boost::chrono 和 std::chrono。使用一个或另一个。如果您的平台提供并支持 std::chrono，我推荐它，否则使用 boost::chrono。

2. 两个time_point之间的区别是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(精度非常低)。 明天 - 今天 == 1 天。一天是一段持续时间。

3. 如果你想要纳秒级的最终结果，没必要这么努力:

   纳秒 nsec = t1 - t0;

4. 如何从 a::time() 返回 nanoseconds 而不是 int？这样，客户端就有了一个类型安全的持续时间，而不是一个可能意味着任何东西的 int。

   返回 t1 - t0;