PRB: C2440 When Initializing an Object Within an Object

• Microsoft Visual C++ for Windows, versions 1.0, 1.5
• Microsoft Visual C++ 32-bit Edition, versions 1.0, 2.0, 4.0, 4.1, 4.2, 5.0

SYMPTOMS

If an object is used to initialize an embedded structure, the compiler produces error C2440: 'initializing': cannot convert from '<type1> ' to

'<type2> '.

CAUSE

This behavior is by design. Although an aggregate that is a class may be initialized with an object of its class, there is not a similar provision for subaggregates.

RESOLUTION

To work around this behavior, use a member by member initialization.

MORE INFORMATION

The following sample code demonstrates this problem and its resolution. The error produced in the initialization of two2 is error C2440: 'initializing' : cannot convert from 'struct ::One ' to 'int '.

Sample Code

   /* Compile options needed: none
   */

   struct One
   {
      int  i;
   };

   struct Two
   {
      int  j;
      One  one;
      int  k;
   };

   One one1 = { 3 };              // Initialize structure member - OK

   One one2 = one1;               // Initialize structure - OK

   Two two1 = { 1, one1, 3 };     // Error, can not initialize subaggregate

   Two two2 = { 1, one1.i, 3 };   // Member by member initialization - OK