How to create a function to be used in an std::ostream or std::cout

Question

Is there a way to create a function which you can use between two << operators in an ostream?

Let's assume the function's name is usd , and might look something like:

std::ostream& usd(std::ostream& os, int value) {
    os << "$" << value << " USD";
    return os;
}

Then I would like to use it like:

int a = 5;
std::cout << "You have: " << usd(a) << std::endl;

Which would print:

You have: $5 USD

I would prefer a solution without the need for a class. If you must use a class I would prefer not to mention the class at all when using the usd function. (For example how the std::setw function works)

EDIT :
In my implementation I intend to use the std::hex function, the one described above was just a simplified example but probably shouldn't have.

std::ostream& hex(std::ostream& os, int value) {
    os << "Hex: " << std::hex << value;
    return os;
}

So I am not sure if a function returning a simple string is sufficient.

Answer 1

To obtain the usage you described:

int a = 5;
std::cout << "You have: " << usd(a) << std::endl;

You'd simply need usd(a) to return something that you have an ostream<< operator for, like a std::string , and no custom ostream<< operator is needed.

For example:

std::string usd(int amount)
{
    return "$" + std::to_string(amount) + " USD";
}

You can write other functions to print in other currencies, or to convert between them, etc but if all you want to handle is USD, this would be sufficient.

---

If you used a class representing money, you could write an ostream<< for that class and you wouldn't need to call a function at all (given that your default ostream<< prints USD)

class Money
{
    int amount;
};

std::ostream& usd(std::ostream& os, Money value) {
    os << "$" << value.amount << " USD";
    return os;
}

int main(int argc, char** argv)
{
    Money a{5};
    std::cout << "You have: " << a << std::endl; // Prints "You have: $5 USD"
    return 0;
}

Answer 2

I don't know how to do this without a class. However, it is easy to do with a class.

struct usd {
    int value;
    constexpr usd(int val) noexcept : value(val) {}
};

std::ostream& operator<<(std::ostream& os, usd value) {
    os << "$" << value.value << " USD";
    return os;
}

for hex

struct hex {
    int value;
    constexpr hex(int val) noexcept : value(val) {}
};

std::ostream& operator<<(std::ostream& os, hex value) {
    os << "Hex: " << std::hex << value.value;
    return os;
}

usage

int a = 5;
std::cout << "You have: " << usd(a) << std::endl;
std::cout << "You have: " << hex(a) << std::endl;