INFO: C and C++ Differences Regarding the Return Statement

• The Microsoft C/C++ Compiler (CL.EXE) included with: - Microsoft C/C++ for MS-DOS, version 7.0 - Microsoft Visual C++ for Windows, versions 1.0, 1.5, 1.51, 1.52 - Microsoft Visual C++, 32-bit Edition, versions 1.0, 2.0, 2.1, 4.0, 5.0

SUMMARY

With both C and C++, the return statement terminates the function currently executing and returns control back to its caller. However, the C and C++ languages differ slightly in their requirement that all paths of control through a function return a value of the correct type.

MORE INFORMATION

The return statement has the form:

   return [expression];

   return void;

The C++ language requires that all possible control paths through a function return a value that is the same type as the return type specified by the function prototype. If this is not the case, a fatal error is generated by the compiler. The C language is not as strict in this requirement. In C, if a control path ends in a return statement that returns a value that is of a different type than that specified in the function prototype, a warning may be generated but compilation will continue.

Section r.6.6.3 of the C++ reference manual provided as a subsection of the second edition of "The C++ Programming Language" by Bjarne Stroustrup states:

   A return statement without an expression can be used only in
   functions that do not return a value, that is, a function with the
   return value type void, a constructor, or a destructor. A return
   statement with an expression can be used only in functions
   returning a value; the value of the expression is returned to the
   caller of the function. ... Flowing off the end of a function is
   equivalent to a return with no value; this is illegal in a value-
   returning function.

   If a return statement without an expression is executed, and the
   value of the function call is used by the caller, the behavior is
   undefined.

   error C2202: 'Func' : not all control paths return a value

Sample Code

   /* Compile options needed: /c /W4
   */

   int Func ( int );

   int Func ( int nParam )
   {
        if ( nParam )
             return 1;
        else
             ;
   }

However, the C compiler sees that there is a return statement present that does return the correct type. Because of this, the compiler does not issue a warning. If no return statement is present at all, the C compiler generates the warning:

   warning C4035: 'Func' : no return value

With Microsoft C++, the only exception to the requirement that every execution path in a C++ file return a value of the correct type is the main() function.

If the sample code given below is compiled as a C++ file (.CPP), the following warning is generated by the compiler:

   warning C4508: 'main' : function should return a value; 'void'
                           return type assumed

   warning C4035: 'main' : no return value

Sample Code

   /* Compile options needed: /c /W3
   */

   int main ( void );

   int main ( void )
   {
   }