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297. const_mem_fun_t<>::argument_type should be const T*Section: 22.10.10 [logical.operations] Status: CD1 Submitter: Martin Sebor Opened: 2001-01-06 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
The class templates const_mem_fun_t in 20.5.8, p8 and const_mem_fun1_t in 20.5.8, p9 derive from unary_function<T*, S>, and binary_function<T*, A, S>, respectively. Consequently, their argument_type, and first_argument_type members, respectively, are both defined to be T* (non-const). However, their function call member operator takes a const T* argument. It is my opinion that argument_type should be const T* instead, so that one can easily refer to it in generic code. The example below derived from existing code fails to compile due to the discrepancy:
template <class T> void foo (typename T::argument_type arg) // #1 { typename T::result_type (T::*pf) (typename T::argument_type) const = // #2 &T::operator(); }
struct X { /* ... */ };
int main () { const X x; foo<std::const_mem_fun_t<void, X> >(&x); // #3 }
#1 foo() takes a plain unqualified X* as an argument
#2 the type of the pointer is incompatible with the type of the member function
#3 the address of a constant being passed to a function taking a non-const X*
Proposed resolution:
Replace the top portion of the definition of the class template const_mem_fun_t in 20.5.8, p8
template <class S, class T> class const_mem_fun_t : public unary_function<T*, S> {
with
template <class S, class T> class const_mem_fun_t : public unary_function<const T*, S> {
Also replace the top portion of the definition of the class template const_mem_fun1_t in 20.5.8, p9
template <class S, class T, class A> class const_mem_fun1_t : public binary_function<T*, A, S> {
with
template <class S, class T, class A> class const_mem_fun1_t : public binary_function<const T*, A, S> {
Rationale:
This is simply a contradiction: the argument_type typedef, and the argument type itself, are not the same.
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