array comparison template operator== does not apply

Asked Jul 22 '18 at 14:20

Active Jul 22 '18 at 14:38

Viewed 67 times

I have written a nice comparison template for c-style arrays as seen below. The only problem is: the operator== is not applied (not found?) by the compiler in the following simple code:

int Array1[3];
int Array2[3];
bool is_equal = ( Array1 == Array2 );

Here is my template

template<typename T, size_t N>
bool operator==( const T ( &A )[N], const T ( &B )[N] )
{
    for ( int i = 0; i < N; ++i )
    {
        if ( A[i] != B[i] )
        {
            return false;
        }
    }
    return true;
}

If I rename the operator==(...) to Equals(...) everything works fine. Why is this???

I also am unsure about the placement of const in the function. Obviously, I want to use 'const &' as for any good comparison operator.

asked Jul 22 '18 at 14:20

old123987

1

Because you cannot compare arrays in C++. C++ does not work this way. Use either `std::vector` or `std::array`. – Sam Varshavchik Jul 22 '18 at 14:21
10

You cannot overload operators on fundamental types. An overloaded operator must take at least one parameter of class or enum type. – Igor Tandetnik Jul 22 '18 at 14:21
Oops. Strangely enough, there is no warning in VC++. – old123987 Jul 22 '18 at 14:28
Did not realize that `T ( &A )[N]` was a fundamental type. – old123987 Jul 22 '18 at 14:30
That's the thing, you **can** define such operator but it will never be used. It would be nice if someone could explain **why** it works this way. – r3mus n0x Jul 22 '18 at 14:31
@SamVarshavchik: Great Job reading past the first line! – old123987 Jul 22 '18 at 14:33
@r3musn0x - Because to do otherwise would make it really hard to determine the meaning of programs. To the extent that including a different header might change a program's meaning unintentionally. We don't want that. – StoryTeller - Unslander Monica Jul 22 '18 at 14:33
@StoryTeller This problem ALWAYS can occur with operator overloading – old123987 Jul 22 '18 at 14:35
2

@old123987 - Not to that great an extent. If you can't reason about the very basic of statements, writing programs becomes intractable. – StoryTeller - Unslander Monica Jul 22 '18 at 14:36
1

As for not getting a peep from MSVC. That's a QoI issue. [GCC](http://coliru.stacked-crooked.com/a/e423932788d69da7) is quite clear on this, for example. – StoryTeller - Unslander Monica Jul 22 '18 at 14:38
1

@StoryTeller Thx. Should have tried this myself... – old123987 Jul 22 '18 at 14:41

array comparison template operator== does not apply

0 Answers0