Visual Studio 2010 (C++): suppress C4706 warning temporarily

Question

When you compile the following C++ source file in Visual Studio 2010 with warning level /W4 enabled

#include <cstdio>  // for printf
#include <cstring> // for strcmp

char str0[] = "Hello";
char str1[] = "World";

int main()
{
    int result;

    if (result = strcmp(str0, str1)) // line 11
    {
        printf("Strings are different\n");
    }
}

you get the following warning

warning C4706: assignment within conditional expression

for line 11.

I want to suppress this warning exactly at this place. So I tried Google and found this page: http://msdn.microsoft.com/en-us/library/2c8f766e(v=VS.100).aspx

So I changed the code to the following - hoping this would solve the problem:

#include <cstdio>  // for printf
#include <cstring> // for strcmp

char str0[] = "Hello";
char str1[] = "World";

int main()
{
    int result;

#pragma warning(push)
#pragma warning(disable : 4706)
    if (result = strcmp(str0, str1))
#pragma warning(pop)
    {
        printf("Strings are different\n");
    }
}

It didn't help.

This variant didn't help either:

#include <cstdio>  // for printf
#include <cstring> // for strcmp

char str0[] = "Hello";
char str1[] = "World";

int main()
{
    int result;

#pragma warning(push)
#pragma warning(disable : 4706)
    if (result = strcmp(str0, str1))
    {
#pragma warning(pop)
        printf("Strings are different\n");
    }
}

To avoid one further inquiry: I cleaned the solution before each compilation. So this is probably not the fault.

So in conclusion: how do I suppress the C4706 exactly at this place?

Edit Yes, rewriting is possible - but I really want to know why the way I try to suppress the warning (that is documented officially on MSDN) doesn't work - where is the mistake?

Answer 1

In MSDN Libray: http://msdn.microsoft.com/en-us/library/2c8f766e(v=VS.100).aspx, There is the section as follows.

For warning numbers in the range 4700-4999, which are the ones associated with code generation, the state of the warning in effect when the compiler encounters the open curly brace of a function will be in effect for the rest of the function. Using the warning pragma in the function to change the state of a warning that has a number larger than 4699 will only take effect after the end of the function. The following example shows the correct placement of warning pragmas to disable a code-generation warning message, and then to restore it.

So '#pragma warning' only works for an each function/method.

Please see the following code for more detail.

#include <cstdio>  // for printf
#include <cstring> // for strcmp

char str0[] = "Hello";
char str1[] = "World";

#pragma warning(push)
#pragma warning( disable : 4706 )
void func()
{
    int result;
    if (result = strcmp(str0, str1)) // No warning
    {
        printf("Strings are different\n");
    }
#pragma warning(pop)
}

int main()
{
    int result;

    if (result = strcmp(str0, str1)) // 4706 Warning.
    {
        printf("Strings are different\n");
    }
}

Answer 2

Instead of trying to hide your warning, fix the issue it's complaining about; your assignment has a value (the value on the left side of the assignment) that can be legally used in another expression.

You can fix this by explicitly testing the result of the assignment:

if ((result = strcmp(str0, str1)) != 0) 
{
    printf("Strings are different\n");
}

Answer 3

The sane solution is to rewrite the condition to

if( (result = strcmp(str0, str1)) != 0 )

which will inform any C compiler that you really want to assign, and is almost certain to generate the same object code.

Answer 4

There is another solution which avoids the warning: the comma operator.

The main advantage here will be that you don't need parentheses so it's a bit shorter than the !=0 solution when your variable name is short.

For example:

if (result = strcmp(str0, str1), result) 
{
    printf("Strings are different\n");
}

Answer 5

There is a simple construction !! to cast a type to bool. Like this:

if (!!(result = strcmp(str0, str1)))

However, in some cases direct comparison != 0 might be more clear to a reader.