c++ - Why does this explicit conversion operator work with g++ but not Visual Studio 2013? -

the following example contains 2 templated classes represent degrees , radians explicit conversion operator cast between them. compiles , runs g++ (ideone link) not visual studio 2013 visual c++ compiler nov 2013 ctp (ctp_nov2013) platform toolset.

#include <iostream>  static const double pi = 3.14159265358979323846;  // forward declarations template< typename t > class radians; template< typename t > class degrees;  template< typename t > class degrees {     public:         degrees(const t value)             : value_(value)         {}          template< typename u >         explicit operator u() const         {             return value_ * pi / 180.0;         }          t value() const { return value_; }      private:         t value_; };  template< typename t > class radians {     public:         radians(const t value)             : value_(value)         {}          template< typename u >         explicit operator u() const         {             return (value_* 180.0) / pi;         }          t value() const { return value_; }      private:         t value_; };  template< typename t > std::ostream& operator<<(std::ostream& out, const radians<t>& r) {     return out << r.value() << "r"; }  template< typename t > std::ostream& operator<<(std::ostream& out, const degrees<t>& r) {     return out << r.value() << "d"; }   int main() {     using degs = degrees<float>;     using rads = radians<float>;      auto d = degs{10};     auto r = static_cast<rads>(d);      std::cout << d << std::endl;     std::cout << r << std::endl;      return 0; } 

the visual studio error output:

error c2440: 'static_cast' : cannot convert 'degrees<float>' 'rads' degrad.cpp  69  1   degrad error c3536: 'r': cannot used before initialized   degrad.cpp  72  1   degrad 

what's wrong? why work g++ not visual studio 2013? compiler doing right thing?

a compiler not accepting metioned snippet faulty, code provided legal , should not yield fatal diagnostic during compilation. in other words; msvc doing wrong.

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relevant sections of standard

12.3.2 conversion functions [class.conv.fct]

² function may explicit (7.1.2), in case considered user-defined conversion direct-initialization (8.5). otherwise, user-defined conversions not restricted use in assignments , initializations.

8.5 initializers [dcl.init]

¹⁶ initialization occurs in forms

t x (a);
t x {a};

as in new expressions (5.3.4), static_cast expressions (5.2.9), functional notation type conversions (5.2.3), , base , member initializers (12.6.2) called direct-initialization.

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how work around msvc++ being faulty?

• use typedef radians<float> rads; instead of using, , either;

  • remove explicit conversion function, or;

  • initialize variable using auto r = rads { d } or auto r = rads (d);.