Question regarding const qualifier and constructor

Question

Here is simple program. If I comment constructor, I get an error Just wanted to check what is the reason for this?

t.cc: In function 'int main(int, char**)':                                                                                                                              
t.cc:26: error: uninitialized const 'const_test'


#include <iostream>                                                                                                                                                     

using namespace std;                                                                                                                                                    

class TestPrint                                                                                                                                                         
{                                                                                                                                                                       
public:                                                                                                                                                                 
  //  TestPrint() {}                                                                                                                                                    
  void Print()                                                                                                                                                          
  {                                                                                                                                                                     
    std::cout << "TestPrint" << std::endl;                                                                                                                              
  }                                                                                                                                                                     

  void Print() const                                                                                                                                                    
  {                                                                                                                                                                     
    std::cout << "const TestPrint" << std::endl;                                                                                                                        
  }                                                                                                                                                                     
};                                                                                                                                                                      


int main(int argc, char* argv[])                                                                                                                                        
{                                                                                                                                                                       
  TestPrint normal_test;                                                                                                                                                
  normal_test.Print();                                                                                                                                                  

  const TestPrint const_test;                                                                                                                                           
  const_test.Print();                                                                                                                                                   
}                                                                                                                             

Answer 1

Your code compiles in Microsoft Visual Studio 2008. Perhaps this is a bug with your compiler, what compiler are you using?

Answer 2

A const-qualified object must be initialised where it is defined; either by an initialiser (e.g. const TestPrint const_test = TestPrint();), or by a default constructor. This rule applies to all objects, even if they don't have any data members to initialise.

So without the default constructor, your code is ill-formed; with it, it is fine and the default constructor is used for initialisation.

Answer 3

It is indeed ill-formed. §8.5/9:

If no initializer is specified for an object, and the object is of (possibly cv-qualified) non-POD class type (or array thereof), the object shall be default-initialized; if the object is of const-qualified type, the underlying class type shall have a user-declared default constructor. Otherwise, if no initializer is specified for a nonstatic object, the object and its subobjects, if any, have an indeterminate initial value; if the object or any of its subobjects are of const-qualified type, the program is ill-formed.

Emphasis mine. Any compiler that does not issue a diagnosis for your program is non-compliant (looking at you MSVC). A simpler test:

struct foo {};

int main()
{
    const foo f;
}

The idea is simple: constants need to be initialized to something. If you have no user-defined constructor, you have no initialization.

Answer 4

According to the ISO standard (8.5 [dcl.init] paragraph 9):

If no initializer is specified for an object, and the object is of (possibly cv-qualified) non-POD class type (or array thereof), the object shall be default-initialized; if the object is of const-qualified type, the underlying class type shall have a user-declared default constructor.

So GCC is right here. Sorry, VC guys.