[SOLVED] Print deduced template arguments at compile time

Issue

How to print automatically deduced template arguments at compile time?

std::pair x(1, 2.0);

In the above example the type of x is std::pair<int, double>. Is there a way to print the deduced type of x at compile time, e.g. as std::pair<int, double> or in some other readable form?

Edit. I am using a library (DPC++) that relies on C++ 17 automatic template argument deduction. Sometimes it is difficult to guess how template argument were deduces.

Solution

Here's one (admittedly roundabout and verbose) approach:

#include <utility>

template <class T>
struct type_reader {
    type_reader() {
        int x{0};
        if (x = 1) {} // trigger deliberate warning
    }
};

int main() {
    std::pair x(1, 2.0);
    type_reader<decltype(x)> t;
}

Depending on your compiler and settings, you can get a warning like

<source>:7:19: warning: using the result of an assignment as a condition without parentheses [-Wparentheses]
        if (x = 1) {} // trigger deliberate warning
<source>:13:34: note: in instantiation of member function 'type_reader<std::pair<int, double>>::type_reader' requested here
    type_reader<decltype(x)> t;

and you can see the type std::pair<int, double>> in that mess if you squint.

Answered By - Ayjay