How to detect if operator[] applicable to Type?

Question

I would like to write function template like

template< typename T >
void foo( T& obj ){
    obj[0] = xxxxxx;
}

where T must have operator[] applicable.
T may be array of any type, std::vector, std::array, or any other type. So, I cannot use T as superclass of all of them. I think it should be something like in std::type_traits style.

Nothing wrong, but if it possible I want to restrict wrong types. — kyb, May 01 '16 at 06:45
@kyb Well, these will be restricted (unless you want to exclude plain pointer types). Do you want better error messages? — πάντα ῥεῖ, May 01 '16 at 06:50
What do you want the result to be if someone calls `foo(1)`? As written, it's a compile-time error, which is, superficially, the appropriate response. — Pete Becker, May 01 '16 at 12:33
You are right, but I looked for something like T.C.'s answer. thank you — kyb, May 01 '16 at 19:24

score 3 · Accepted Answer · answered May 01 '16 at 06:55

template<class T>
using LvalueIndexable = decltype(std::declval<T&>()[1]);

template<class T, class U = void>
using RequiresLvalueIndexable 
    = typename std::enable_if<std::experimental::is_detected<LvalueIndexable, T>{},
                              U>::type;

template< typename T, typename = RequiresLvalueIndexable<T> >
void foo( T& obj ){
    obj[0] = xxxxxx;
}

See the cppreference page for how to implement std::experimental::is_detected.

score 1 · Answer 2 · edited May 23 '17 at 12:07

1

There are several ways to restrict template types:

1) declare the function template as a private class method and then call it from public overloaded methods, as described here;

2) using Boost static assert and is_base_of to compare template and types, see here;

3) or include type_traits and use asserts static_assert(is_same<T, float>::value, "Error message");

edited May 23 '17 at 12:07

Community

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answered May 01 '16 at 07:08

FedeWar

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How to detect if operator[] applicable to Type?

2 Answers2