c++ - Copy Constructor invoked instead of assignment operator -


consider following self contained code. 

#include <iostream> template<typename ty> class foo { private:     ty m_data; public:     foo() :m_data() {}     foo(ty data) :m_data(data) {}     template<typename u>     foo& operator=(foo<u> rv)     {         m_data = rv.m_data;         return *this;     } private:     foo(foo&);     foo& operator=(foo&); }; int main() {     foo<int> na(10);     foo<int> nb;     nb = foo<int>(10); // (1)     foo<int>(10); // (2) }

my understanding statement (1) assignment rather copy constructor. yet, when compiling (vc++ , g++), error message states, tries match copy constructor declared private. 

1>source.cpp(23): error c2248: 'foo<int>::foo' : cannot access private member declared in class 'foo<int>' 1>          source.cpp(16) : see declaration of 'foo<int>::foo'

my question is, why try search copy constructor instead of assignment.

note, know assignment failing because (2) compiles fine without error.

there two issues:

  1. your private assignment operator foo& operator=(foo&); takes non-const lvalue reference. means cannot selected overload in nb = foo<int>(10);, because rhs rvalue
  2. that leads template assignment operator being selected. takes argument value, requiring copy or move copy constructor.

if fix 1. take const reference, gcc gives following error:

error: 'foo& foo::operator=(const foo&) [with ty = int]' private

if fix 2. template assignment operator takes const reference, code compiles without errors.


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