CODEWITHSUNDEEP
c++templatesconstantstemplate-functiontemplate-argument-deduction
Ryan
Ryan

Reputation: 168

Why does std::is_same give a different result for the two types?

In the code below, why do the two ways of invoking fun: fun(num) and fun<const int>(num), give a different result when compiling?

#include <iostream>
using namespace std;

template<typename T, typename = typename enable_if<!std::is_same<int, T>::value>::type>
void fun(T val)
{
    cout << val << endl;
}

int main(void)
{
    const int num = 42;
    fun(num);  //ERROR!

    fun<const int>(num);  //Right

    return 0;
}

Upvotes: 9

Views: 723

Answers (1)

songyuanyao
songyuanyao

Reputation: 172924

The parameter is declared as pass-by-value; then in template argument deduction, the top-level const qualifier of the argument is ignored.

Before deduction begins, the following adjustments to P and A are made:

1) If P is not a reference type,

a) ...

b) ...

c) otherwise, if A is a cv-qualified type, the top-level cv-qualifiers are ignored for deduction:

So given fun(num), the template parameter T will be deduced as int, not const int.

If you change the parameter to pass-by-reference, the const part will be preserved. e.g.

template<typename T, typename = typename enable_if<!std::is_same<int, T>::value>::type>
void fun(T& val)

Then for fun(num), T will be deduced as const int.

Upvotes: 20

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