CODEWITHSUNDEEP
c++templatestype-conversionimplicit-conversion
eucristian
eucristian

Reputation: 400

Template conversion operator issue

I have done a bit of browsing and this was the most relevant link that I could find, however it does not answer my question

Question: Why does the template substitution fail and the following does not compile?

template <typename T>
struct A
{
   A() {};
   A(T value) : val(value){} 
   operator T() { return this->val;}
   T val;
};


A<std::string> test;
std::cout << "xxx" + std::string(test); //works fine
std::cout << "xxx" + test; //compiler error

Error message:

error: no match for 'operator+' (operand types are 'const char [4]' and 'A<std::__cxx11::basic_string<char> >')
   19 |    std::cout << "xxx" + test;
      |                 ~~~~~ ^ ~~~~
      |                 |       |
      |                 |       A<std::__cxx11::basic_string<char> >
      |                 const char [4]

Upvotes: 2

Views: 400

Answers (1)

songyuanyao
songyuanyao

Reputation: 172894

std::operator+(std::basic_string) is a set of operator templates, template argument deduction needs to be performed on the 2nd operand test. But implicit conversion (from A<std::string> to std::string) won't be considered in template argument deduction.

Type deduction does not consider implicit conversions (other than type adjustments listed above): that's the job for overload resolution, which happens later.

As you have showed, explicit conversion like "xxx" + std::string(test); works fine. You can also specify template arguments explicitly (in ugly way) to bypass template argument deduction.

operator+ <char, std::char_traits<char>, std::allocator<char>>("xxx", test);

Upvotes: 4

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