Output stream for generic container

Question

I have created this template function:

// Output stream for container of type C
template<class C>
ostream& operator<<(ostream& os, const C& c) {
    os << "[";
    for (auto& i : c) {
        os << i;
        if (&i != &c.back()) os << ", ";
    }
    os << "]" << endl;
    return os;
}

but I have this error:

error: use of overloaded operator '<<' is ambiguous (with operand types 'std::ostream' (aka 'basic_ostream') and 'const char [2]')

The error is in the first line of the body of the function.

You really shouldn't provide unconstrained templates like that. You also shouldn't overload operators for types you don't own. If it were me, I would make a generic print or stringify function and use that to print/return a string able to be printed. — NathanOliver
You have provided a brief description of what happens what you compile the shown code: an error message. Your narrative ends at this point, and what seems to be missing from your narrative is an actual question. What is your specific question? — Sam Varshavchik
I noted that if v is a STL container I can't "cout << v" then I want an overload of operator<<. — asv

463035818_is_not_a_number 463035818_is_not_a_number · Accepted Answer · 2019-12-06T13:43:03

This declaration

template<class C>
ostream& operator<<(ostream& os, const C& c)

is a match for any type. In particular, when you call inside that operator

os << "[";
os << i;
os << "]" << endl;

For all this calls your operator is a match in addition to the already existing output operators for strings and endl.

Don't provide operators for types you don't own. Instead you can write a

void print(const my_container&);

or use a tag to resolve ambiguity. For example you could use a

template <typename T>
struct pretty_printed_container {
    const T& container;
};

template <typename T>
pretty_printed_container<T> pretty_print(const T& t) { return {t};}

modify your output operator accordingly

template<class T>
std::ostream& operator<<(std::ostream& os, const pretty_printed_container<T>& c) {
    os << "[";
    for (const auto& i : c.container) {
        os << i;
        if (&i != &c.container.back()) os << ", ";
    }
    os << "]" << std::endl;
    return os;
}

And then use it like this

int main() {
    std::vector<int> x{1,2,3,4,5};
    std::cout << pretty_print(x);
}

Output:

[1, 2, 3, 4, 5]

Output stream for generic container

1 Answers