C++ template with default parameters

Question

I try to use default parameters in my template, like this

#include <iostream>

using namespace std;

template <typename S, typename T=int> 
S myadd(T a, T b)
{
    S tmp = a + b;
    return tmp;
}

int main()
{
    int a = 1, b = 2;
    float i = 5.1, j = 5.2;

    cout << myadd<int, int>(i, j);
    cout << myadd<float>(a, b);

    return 0;
}

Then g++ myadd.cpp

It shows error:

default template arguments may not be used in function templates without -std=c++0x or -std=gnu++0x

Why does this happen?

What is it that you don't understand in that error message? What did you try to make this compile? — Mat, May 17 '12 at 14:05
You don't know why? The why is because "default template arguments may not be used in function templates without -std=c++0x or -std=gnu++0x". — R. Martinho Fernandes, May 17 '12 at 14:05
Just a wild guess, but are you compiling with -std=c++0x or -std=gnu++0x? — Luchian Grigore, May 17 '12 at 14:05
Why would you want a default T in this case? T can be deducted from the parameters' types. — mfontanini, May 17 '12 at 14:06
@LuchianGrigore I just use g++ and g++ myadd.cpp in a terminal. — Jeff Ma, May 17 '12 at 14:14
Possible duplicate of *[Default template arguments for function templates](http://stackoverflow.com/questions/2447458/default-template-arguments-for-function-templates)*. — Peter Mortensen, Jun 23 '15 at 21:02

score 1 · Accepted Answer · edited May 23 '17 at 11:51

I extracted this answer from Stack Overflow question Default template arguments for function templates:

To quote C++ Templates: The Complete Guide (page 207):

When templates were originally added to the C++ language, explicit function template arguments were not a valid construct. Function template arguments always had to be deducible from the call expression. As a result, there seemed to be no compelling reason to allow default function template arguments because the default would always be overridden by the deduced value.

Read the Stack Overflow question to understand quite more. NOTE: the default parameter in a function template C++ is a new feature in gnu++0x.

C++ template with default parameters

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