how to do an if else depending type of type in c++ template? [duplicate]

You could use typeid:

if (typeid(T) == typeid(int))

Or you could use the std::is_same type trait:

if (std::is_same<T, int>::value)

What you want is probably something like a compile-time if. Unfortunately, C++11 has no native support for such a language construct.

However, if you just want to check whether two types are identical, the std::is_same<> type trait should help you:

#include <type_traits> // <== INCLUDE THIS STANDARD HEADER

// class template:
template <class T>
class mycontainer 
{
    T element;
public:
    mycontainer (T arg) {element=arg;}
    T increase () 
    {
        if (std::is_same<T, int>::value)   // <== THIS IS HOW YOU WOULD USE IT
            return ++element;

        if (std::is_same<T, char>::value)  // <== THIS IS HOW YOU WOULD USE IT
        {
            if ((element>='a') && (element<='z'))
                element+='A'-'a';
        }

        return element;
    }
};

However, keep in mind that the condition is evaluated at run-time, even though the value of is_same<T, int>::value is known at compile-time. This means that both the true and the false branch of the if statement must compile!

For instance, the following would not be legal:

if (std::is_same<T, int>::value)
{
    cout << element;
}
else if (std::is_same<T, my_class>::value)
{
    element->print();  // Would not compile when T is int!
}

Also, as Xeo correctly pointed out in the comments, the compiler will likely issue warnings because your condition will always evaluate to true or to false, so one of the two branches will contain unreachable code.

how to do an if else depending type of type in c++ template? [duplicate]

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