PRB: C2440 When Initializing an Object Within an Object (121888)

The information in this article applies to:

Microsoft Visual C++ 1.0
Microsoft Visual C++ 1.5
Microsoft Visual C++ 2.0
Microsoft Visual C++ 4.0
Microsoft Visual C++ 4.1
Microsoft Visual C++, 32-bit Enterprise Edition 4.2
Microsoft Visual C++, 32-bit Enterprise Edition 5.0
Microsoft Visual C++, 32-bit Enterprise Edition 6.0
Microsoft Visual C++, 32-bit Professional Edition 4.2
Microsoft Visual C++, 32-bit Professional Edition 5.0
Microsoft Visual C++, 32-bit Professional Edition 6.0
Microsoft Visual C++, 32-bit Learning Edition 6.0

This article was previously published under Q121888

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

Modification Type:	Major	Last Reviewed:	12/1/2003
Keywords:	kbCompiler kbCPPonly kbprb KB121888