Overload operator<< for arrays

Question

I want to overload operator<< for arbitrary arrays, such that the code cout << my_arr would work. First I tried to overload the second parameter of operator<< on const T (&arr)[N], where T and N are template parameters. But testing the code revealed a side effect: const char[] also matches the type specification, and the new overload conflicts with the one defined in the stream class. Example code:

#include <cstddef>
#include <iostream>

template<typename T, std::size_t N>
std::ostream& operator<<(std::ostream& os, const T (&arr)[N])
{
    /* do stuff */
    return os;
}

int main()
{
    std::cout << "noooo\n"; /* Fails: ambiguous overload */
}

Can such an array printing operator still be implemented?

I don't think N will transfer well in a lot of cases. void f(int arr[], size_t N){ cout<<arr; } — Captain Giraffe
– Captain Giraffe, Commented Feb 23, 2012 at 21:31
if you want an external library why not just use boost.org/doc/libs/1_48_0/doc/html/boost_lexical_cast.html — pyCthon
– pyCthon, Commented Feb 23, 2012 at 21:33
@Captain: arr actually has the type int* in that case, so it wouldn't match that overload. — Georg Fritzsche
– Georg Fritzsche, Commented Feb 23, 2012 at 21:49
@GeorgFritzsche You are correct. A simple test revealed "error: no matching function for call to ‘operator<<(std::ostream&, int*&)’" Thanks! — Captain Giraffe
– Captain Giraffe, Commented Feb 23, 2012 at 21:59

Georg Fritzsche · Accepted Answer · 2012-02-23 21:44:10Z

5

Sure:

template<typename T, std::size_t N>
typename std::enable_if<!std::is_same<T, char>::value, std::ostream&>::type
operator<<(std::ostream& os, const T (&arr)[N])
{
    // ...
}

This will disable your overload when T is char using SFINAE.

For C++03, Boost has enable_if and is_same. Alternatively just roll your own:

template<class T, class U> struct is_same { 
    enum { value = false }; 
};
template<class T> struct is_same<T, T> { 
    enum { value = true }; 
};

template<bool, class T> struct enable_if {};
template<class T> struct enable_if<true, T> { 
    typedef T type; 
};

edited Feb 23, 2012 at 21:44

answered Feb 23, 2012 at 21:33

Georg Fritzsche

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2 Comments

Cat Plus Plus Over a year ago

@Mr.Anubis: Use boost::enable_if and boost::is_same instead. If you don't want Boost, implement them yourself, these two are trivial.

Ben Voigt Over a year ago

@GeorgFritzsche: You're right. But should it be? I guess that const T(&)[] can bind to a non-const array also, so yeah, it's fine as written. It's deeper levels of const that can't be added implicitly.

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