Different results between an integer variable and casting an integer to an integer

Question

I've written a generic timer class that uses the std::chrono time for the timer. This is a code example that shows the issue:

#include <iostream>
#include <chrono>

template <typename Rep, typename TimeType>
class Timer {
  public:
    Timer(const std::chrono::duration<Rep, TimeType> timerLength);
    ~Timer() = default;
  private:
    std::chrono::duration<Rep, TimeType> _timerLength;
};

template <typename Rep, typename TimeType>
Timer<Rep, TimeType>::Timer(const std::chrono::duration<Rep, TimeType> timerLength) : _timerLength{timerLength} {}

int main()
{
    constexpr int time_ms = 1000;

    Timer timer(std::chrono::milliseconds(time_ms));

    return 0;
}

This code results in the error: error: deduced class type ‘Timer’ in function return type.

This error can be removed by placing an actual int directly in the timer in main:

Timer timer(std::chrono::milliseconds(1000));

Additionally, this error can be removed by casting the time_ms parameter to an integer:

Timer timer(std::chrono::milliseconds(`static_cast<int>(time_ms)));

I expected the same result of an integer variable and casting an integer to an integer.

This error cannot be removed by removing the constexpr of the time_ms variable.

My question is: What are the differences between an integer variable and casting an integer to an integer (as well as inserting the number directly in the std::chrono::milliseconds())? And why does it make a difference in the first place?

Does this answer your question? [Most vexing parse](https://stackoverflow.com/questions/38951362/most-vexing-parse) — JaMiT, Apr 02 '20 at 14:02
`{}` will save you from the Most Vexing Parse: `Timer timer{std::chrono::milliseconds(time_ms)}`. — Howard Hinnant, Apr 02 '20 at 14:03

cigien · Accepted Answer · 2020-04-02T14:08:16.550

You are a victim of a vexing parse. With -Wall clang will even warn about that

source>:24:16: warning: parentheses were disambiguated as a function declaration [-Wvexing-parse]

    Timer timer(std::chrono::milliseconds(time_ms));

               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

and suggests a fix as well.

<source>:24:17: note: add a pair of parentheses to declare a variable

    Timer timer(std::chrono::milliseconds(time_ms));

                ^

                (                                 )

But you can disambiguate in other ways as well.

Timer timer{std::chrono::milliseconds(time_ms)};
Timer timer(std::chrono::milliseconds{time_ms});
auto timer = Timer(std::chrono::milliseconds(time_ms));
Timer timer(static_cast<std::chrono::milliseconds>(time_ms));

as well as putting an int literal in the expression, or explicitly casting to an int, as you noticed yourself.

Different results between an integer variable and casting an integer to an integer

1 Answers1