如何正确重载算术运算符，以便在C ++中“ 3” +“ 4”可以为“ 7”

Question

I overloaded + with

string operator+(string a, string b);

it works when I do this:

string a = "3";
string b = "4";
cout << a + b;

However when I do this, it fails and with error message: invalid operand to binary expression(const char * and const char* )

cout << "3" + "4";

Can someone tell me how to solve this problem?

Thanks for reply, I see why there is an error, I shouldn't overload operator in c++ which operand is built-in type.

Answer 1

You can't overload an operator unless you make at least one of the arguments a user defined type, so what you ask is impossible. However, the nearest you could come to that syntax is to perhaps use user defined literals. In C++14, the s suffix is defined to create a std::string .

using namespace std::string_literals;
std::cout << "3"s + "4"s;

If you can't use C++14, but can use C++11, then you can define your own literal suffix.

std::string operator""_s (const char* cs, size_t)
{
    return std::string(cs);
}