Accessing a class member in an if statement using std::is_same

Question

I\'m trying to tackle the following problem: I would like to do an if statement that does something depending on whether the argument of a template is a specific ob

Answer 1

In a regular if statement, both branches must be valid code. In your case int.length() makes no sense.

In C++17 you could simply use constexpr if:

if constexpr(std::is_same<T, const std::string&>::value)
    std::cout << arg.length() << std::endl;
else
    std::cout << "The argument is not a string" << std::endl;

demo

In C++11 (or older) you can employ overloading to achieve similar result:

void foo(std::string const& str){
    std::cout << str.length() << std::endl;
}

template<typename T>
void foo(T const&){
    std::cout << "The argument is not a string" << std::endl;
}

template <typename T>
void is_string(const T& arg) {
    foo(arg);
}

demo

Answer 2

void is_string(const std::string& arg) {
  std::cout << arg.length() << std::endl;
}

template <typename T>
void is_string(const T& arg) {
  std::cout << "The argument is not a string" << std::endl;
}

Or, see if your compiler supports the C++17 if constexpr.