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Compiler Warning (level 4) C4706

Error Message

assignment within conditional expression

The test value in a conditional expression was the result of an assignment.

An assignment has a value (the value on the left side of the assignment) that can be used legally in another expression, including a test expression.

The following sample generates C4706:

// C4706a.cpp
// compile with: /W4
int main()
{
   int a = 0, b = 0;
   if ( a  = b ) // C4706
   {
   }
}

The warning will occur even if you double the parentheses around the test condition:

// C4706b.cpp
// compile with: /W4
int main()
{
   int a = 0, b = 0;
   if ( ( a  =  b ) ) // C4706
   {
   }
}

If your intention is to test a relation and not to make an assignment, use the == operator. For example, the following line tests whether a and b are equal:

// C4706c.cpp
// compile with: /W4
int main()
{
   int a = 0, b = 0;
   if ( a == b )
   {
   }
}

If you intend to make your test value the result of an assignment, test to ensure that the assignment is non-zero or not null. For example, the following code will not generate this warning:

// C4706d.cpp
// compile with: /W4
int main()
{
   int a = 0, b = 0;
   if ( ( a = b ) != 0 )
   {
   }
}
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