How can you check whether a templated class has a member function?

Asked 2008-11-02T20:10:47.673

644

Is it possible to write a template that changes behavior depending on if a certain member function is defined on a class?

Here's a simple example of what I would want to write:

template<class T>
std::string optionalToString(T* obj)
{
    if (FUNCTION_EXISTS(T->toString))
        return obj->toString();
    else
        return "toString not defined";
}

So, if class T has toString() defined, then it uses it; otherwise, it doesn't. The magical part that I don't know how to do is the "FUNCTION_EXISTS" part.

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2 Answers

Here are some usage snippets: *The guts for all this are farther down

Check for member x in a given class. Could be var, func, class, union, or enum:

CREATE_MEMBER_CHECK(x);
bool has_x = has_member_x<class_to_check_for_x>::value;

Check for member function void x():

//Func signature MUST have T as template variable here... simpler this way :\
CREATE_MEMBER_FUNC_SIG_CHECK(x, void (T::*)(), void__x);
bool has_func_sig_void__x = has_member_func_void__x<class_to_check_for_x>::value;

Check for member variable x:

CREATE_MEMBER_VAR_CHECK(x);
bool has_var_x = has_member_var_x<class_to_check_for_x>::value;

Check for member class x:

CREATE_MEMBER_CLASS_CHECK(x);
bool has_class_x = has_member_class_x<class_to_check_for_x>::value;

Check for member union x:

CREATE_MEMBER_UNION_CHECK(x);
bool has_union_x = has_member_union_x<class_to_check_for_x>::value;

Check for member enum x:

CREATE_MEMBER_ENUM_CHECK(x);
bool has_enum_x = has_member_enum_x<class_to_check_for_x>::value;

Check for any member function x regardless of signature:

CREATE_MEMBER_CHECK(x);
CREATE_MEMBER_VAR_CHECK(x);
CREATE_MEMBER_CLASS_CHECK(x);
CREATE_MEMBER_UNION_CHECK(x);
CREATE_MEMBER_ENUM_CHECK(x);
CREATE_MEMBER_FUNC_CHECK(x);
bool has_any_func_x = has_member_func_x<class_to_check_for_x>::value;

CREATE_MEMBER_CHECKS(x);  //Just stamps out the same macro calls as above.
bool has_any_func_x = has_member_func_x<class_to_check_for_x>::value;

Details and core:

answered 2011-06-12T21:40:44.083

Now this was a nice little puzzle - great question!

Here's an alternative to Nicola Bonelli's solution that does not rely on the non-standard typeof operator.

Unfortunately, it does not work on GCC (MinGW) 3.4.5 or Digital Mars 8.42n, but it does work on all versions of MSVC (including VC6) and on Comeau C++.

The longer comment block has the details on how it works (or is supposed to work). As it says, I'm not sure which behavior is standards compliant - I'd welcome commentary on that.

update - 7 Nov 2008:

It looks like while this code is syntactically correct, the behavior that MSVC and Comeau C++ show does not follow the standard (thanks to Leon Timmermans and litb for pointing me in the right direction). The C++03 standard says the following:

14.6.2 Dependent names [temp.dep]

Paragraph 3

In the definition of a class template or a member of a class template, if a base class of the class template depends on a template-parameter, the base class scope is not examined during unqualified name lookup either at the point of definition of the class template or member or during an instantiation of the class template or member.

So, it looks like that when MSVC or Comeau consider the toString() member function of T performing name lookup at the call site in doToString() when the template is instantiated, that is incorrect (even though it's actually the behavior I was looking for in this case).

The behavior of GCC and Digital Mars looks to be correct - in both cases the non-member toString() function is bound to the c

answered 2008-11-04T08:37:40.677

How can you check whether a templated class has a member function?

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