How to get a compiler error for assignments in an if statement?

Question

Is it possible in Visual Studio to get a compiler error for an assignment in an if statement? How?

#include <iostream>
int main()
{
    int a = 2;
    if (a = 3)  // Want a warning here
        std::cout << "Avoid this!\n";
}

I know I can switch to Yoda conditions (if (3=a)), but I really don't want to.

I tried: setting the warning level to /Wall but I still don't get a warning that I could then treat as an error.

I am doing this in Visual Studio 2019 (16.11.19).:

The build output is

Rebuild started...
1>------ Rebuild All started: Project: Yoda2019, Configuration: Debug Win32 ------
1>Yoda2019.cpp
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\limits.h(70,5): error C2220: the following warning is treated as an error
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\limits.h(70,5): warning C4668: '__STDC_WANT_SECURE_LIB__' is not defined as a preprocessor macro, replacing with '0' for '#if/#elif'
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\xmemory(154,5): warning C4365: 'argument': conversion from 'long' to 'unsigned int', signed/unsigned mismatch
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\xmemory(164,5): warning C4365: 'argument': conversion from 'long' to 'unsigned int', signed/unsigned mismatch
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\atomic(284,9): warning C4365: 'argument': conversion from 'long' to 'unsigned int', signed/unsigned mismatch
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\atomic(299,9): warning C4365: 'argument': conversion from 'long' to 'unsigned int', signed/unsigned mismatch
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\atomic(315,9): warning C4365: 'argument': conversion from 'long' to 'unsigned int', signed/unsigned mismatch
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\atomic(375,9): warning C4365: 'argument': conversion from 'long' to 'unsigned int', signed/unsigned mismatch
1>Done building project "Yoda2019.vcxproj" -- FAILED.
========== Rebuild All: 0 succeeded, 1 failed, 0 skipped ==========

and Visual Studio 2022 (17.4.0)

The build output is

Rebuild started...
1>------ Rebuild All started: Project: YodaCheck, Configuration: Debug x64 ------
1>YodaCheck.cpp
1>YodaCheck.vcxproj -> B:\Projekte\C++\Dynamic Linking\YodaCheck\x64\Debug\YodaCheck.exe
========== Rebuild All: 1 succeeded, 0 failed, 0 skipped ==========
========== Elapsed 00:01,379 ==========

so I see no warning which I could convert into an error.

Most compilers will issue a warning, just turn warnings into errors using your specific compile flags — πάντα ῥεῖ
– πάντα ῥεῖ, Commented Oct 9, 2022 at 13:31
Isn't warning C4706: assignment within conditional expression suitable? — Eljay
– Eljay, Commented Oct 9, 2022 at 13:31
@πάνταῥεῖ: I heard people say so, yes. My question is to how actually do that in Visual Studio. — Thomas Weller
– Thomas Weller, Commented Oct 9, 2022 at 13:33
@ThomasWeller in VS 2022 you're configuring Debug Win32 but building Debug x64. In VS 2019 the build fails so it didn't come to the warning stage for that line. Why do you even build Win32 these days? — phuclv
– phuclv, Commented Oct 9, 2022 at 13:52

Thomas Weller · Accepted Answer · 2022-10-09 14:07:02Z

7

You can turn any specific warning into an error, using the #pragma warning(error:nnnn) directive. In your case, the warning is:

warning C4706: assignment within conditional expression

So, adding the relevant #pragma directive to the code will generate a compiler error:

#include <iostream>
#pragma warning(error:4706)

int main()
{
    int a = 2;
    if (a = 3)  // Want a warning here
        std::cout << "Avoid this!\n";
}

This now gives:

error C4706: assignment within conditional expression

For a solution wide setting, change it here in the property pages:

edited Oct 9, 2022 at 14:07

Thomas Weller

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answered Oct 9, 2022 at 13:37

Adrian Mole

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2 Comments

Adrian Mole Over a year ago

Note that this directive will generate an error whatever the current warning level is set to in the command-line/project options. Even with /W1, I still get the error in VS2022.

Thomas Weller Over a year ago

Thank you very much. I edited your answer to include a screenshot on how to make a solution wide setting. The main problem for me was my selection of configuration and platform.

Thomas Weller · Accepted Answer · 2022-10-09 14:03:44Z

7

Use /W4 /WX

/WX Treats all compiler warnings as errors. For a new project, it may be best to use /WX in all compilations; resolving all warnings ensures the fewest possible hard-to-find code defects.

https://learn.microsoft.com/en-us/cpp/build/reference/compiler-option-warning-level?view=msvc-170

Make sure you make that setting in the relevant configurations. In such a simple demo, it's probably easiest to choose "all configurations" and "all platforms".

edited Oct 9, 2022 at 14:03

Thomas Weller

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answered Oct 9, 2022 at 13:32

phuclv

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7 Comments

Eljay Over a year ago

Or for just C4706 in particular, #pragma warning( error : 4706 ) is an option.

Thomas Weller Over a year ago

I have done this already. See my screenshots in the question. As I said, I don't get a warning for that, so there's nothing to be converted into an error.

MatG Over a year ago

@ThomasWeller I don't see the switch /W4 in your question

Thomas Weller Over a year ago

@MatG: So W4 is more than Wall?

phuclv Over a year ago

no, /Wall is higher, but it includes so many redundant warnings so /W4 is more commonly used

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