No match for "operator =" when using std::chrono::high_resolution_clock

Question

When I compiled this timer.hpp header file below, the compiler said:

error: no match for ‘operator=’ (operand types are ‘std::chrono::_V2::system_clock::time_point {aka std::chrono::time_point > >}’ and ‘std::__success_type > >::type {aka std::chrono::duration >}’) end = std::chrono::high_resolution_clock::now() - start;

I guess the variable type for start and end is wrong. What is the correct type? I want to use std::chrono::high_resolution_clock.

#include <chrono> namespace timer{ static std::chrono::system_clock::time_point start, end; void initTime(){ start = std::chrono::high_resolution_clock::now(); } void endTime(){ end = std::chrono::high_resolution_clock::now() - start; } }

timer.hpp is supposed to be used with some main file.
By calling timer::initTime() before some function that I want to measure and calling timer::endTime() after the function, I would get the timing result (the getter for the duration time is omitted here).

Here is a video tutorial for chrono: youtube.com/watch?v=P32hvk8b13M It addresses issues such as these. — Howard Hinnant
– Howard Hinnant, Commented Jun 22, 2017 at 15:58

NicholasM · Accepted Answer · 2019-07-06 04:01:30Z

There are two problems with this code:

static std::chrono::system_clock::time_point start, end; /* ... */ void endTime(){ end = std::chrono::high_resolution_clock::now() - start; }

You declare end as a time point, but then on the right-hand side of the assignment operator, you are subtracting two time points (now() and start), and assigning to end.

Logically, if you subtract two time points, you do not obtain a new time point. For instance, if I wanted to subtract "08:15:00 today" - "08:05:00 today", it would not make sense to describe the result as "00:10:00 today". Instead, the C++ chrono library has a duration class template; it is intended to represent lengths of time (e.g. the difference between two time points).

See the operator - overload number 4 here: http://en.cppreference.com/w/cpp/chrono/time_point/operator_arith2

I suggest watching the tutorial video that @Howard Hinnant linked to above... Mr. Hinnant was involved in developing the std::chrono and boost::chrono libraries.

A potential second issue is that start has type std::chrono::system_clock::time_point, which may be a different type (different clock) than the type returned by std::chrono::high_resolution_clock::now() (which has type std::chrono::high_resolution_clock::time_point).

Nitpick: I'm the lead author (there were other contributors) on the std::chrono library which served as the basis for the boost::chrono library. :-)
Thanks. One of my points is that in this case, std came first and boost followed. Well, at least the draft std came first. The boost lib probably came about before the std went final in 2011. This isn't really important for your good answer (which I upvoted). These are just historical tidbits.
"08:15:00 today" - "08:05:00 today" = "00:10:00 01/01/1970", not "00:10:00 today" :P
Quick question, say I have an instance variable of type std::chrono::time_point<std::chrono::system_clock> timestamp. How do I reassign this variable? You are unable to reassign it due to chrono::time_point not having an operator= method in its definition. AKA the following doesn't work: timestamp = std::chrono::system_clock::now() Due to the following msg: operand types are: const std::chrono::time_point<std::chrono::_V2::system_clock, std::chrono::_V2::system_clock::duration> = std::chrono::_V2::system_clock::time_point

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