C++ function that returns string, float or int

Question

So I am over simplifying what I am trying to do here, but basically I have a function that looks like this:

int perform_operation(int left, std::string op, int right) {
    if (op == "+")
        return left + right;
    if (op == "-")
        return left - right;

    ... 

};

I want this function to be able to take in float , int , and string as left and right arguments. If strings is passed in and the + operator is used, the strings should be concatenated, if a operator that does not support strings, it should throw an error.

I also want the function to be able to return both float , int and string .

Maybe this is impossible, if so please give me an advice on how to do this instead.

...

I am writing an interpreter if anyone is wondering.

Answer 1

You can achieve this with a function template.

template<class T>
T perform_operation(const T& left, std::string_view op, const T& right)
{
    if (op == "+")
        return left + right;
    if (op == "-")
        return left - right;

    // ...
}

Now as std::string does not support operator - and you want the operation to throw an error, you need to specialize the template for this type:

template<>
std::string perform_operation<std::string>(const std::string& left, std::string_view op, const std::string& right)
{
    if (op == "+")
        return left + right;

    throw std::invalid_argument("std::string supports operator + only");
}

This can be instantiated and invoked like the following.

const int result1 = perform_operation(1, "+", 2);
const float result2 = perform_operation(2.f, "-", 3.f);
const std::string result3 = perform_operation<std::string>("hello", "+", " world");

assert(result1 == 3);
assert(std::abs(result2 + 1.0f) < std::numeric_limits<float>::epsilon()));
assert(result3 == "hello world");

Note that I have changed the argument types to accept the operands as const -qualified references and the operator as std::string_view (a C++17 feature), but the latter isn't required.

Answer 2

Not sure why the question is downvoted, because this makes perfect sense in C++.

What you need is a template . Specifically, a function template.

template <typename T>
T perform_operation(T left, std::string op, T right) {
    if (op == "+")
        return left + right;
    if (op == "-")
        return left - right;
    // ... 
}

Of course, there's no operator- for templates, so you can use an overload:

std::string perform_operation(std::string left, std::string op, std::string right) {
    if (op == "+")
        return left + right;
   // ...
}