Compilation error with MSVC since 2.5.0

[addy90]

Hi there,

there is a compilation error of csv-parser since version 2.5.0 with MSVC, see this example: https://godbolt.org/z/jea4fjf8e

example.cpp
C:\Windows\TEMP\compiler-explorer-compiler8BP4DR\csv.hpp(3280): error C2039: 'result_of': is not a member of 'std'
Z:\compilers\msvc\14.44.35207-14.44.35219.0\include\unordered_set(23): note: see declaration of 'std'
C:\Windows\TEMP\compiler-explorer-compiler8BP4DR\csv.hpp(3280): error C2653: 'result_of': is not a class or namespace name
C:\Windows\TEMP\compiler-explorer-compiler8BP4DR\csv.hpp(3280): error C2061: syntax error: identifier 'type'
Compiler returned: 2

The error was introduced with #290

Replaced std::result_of (removed in C++20) with a csv::internals::invoke_result_t alias that selects std::invoke_result on C++17+ and std::result_of on older standards.

The issue arises from this line:

csv-parser/include/internal/common.hpp

Line 133 in 503a0ec

using invoke_result_t = typename std::result_of<F(Args...)>::type;

The problem comes from here:

csv-parser/include/internal/common.hpp

Line 80 in 503a0ec

#if (defined(CMAKE_CXX_STANDARD) && CMAKE_CXX_STANDARD == 17) || __cplusplus >= 201703L

The issue is that MSVC sets __cplusplus to 199711L by default, unless /Zc:__cplusplus is supplied: https://learn.microsoft.com/en-us/cpp/build/reference/zc-cplusplus?view=msvc-170

One idea to fix this issue is to use _MSVC_LANG in addition, see: https://learn.microsoft.com/en-us/cpp/preprocessor/predefined-macros?view=msvc-170
(search for _MSVC_LANG)

One solution could be to use this code instead for the lines starting from

csv-parser/include/internal/common.hpp

Line 57 in e26bb30

#if (defined(CMAKE_CXX_STANDARD) && CMAKE_CXX_STANDARD == 20) || __cplusplus >= 202002L

Here is the provided solution, it also takes into account when no CMake is used and the CMAKE_CXX_STANDARD variable is not set:

#if defined(CMAKE_CXX_STANDARD)
    #define CSV_CMAKE_CXX_STANDARD CMAKE_CXX_STANDARD
#else
    #define CSV_CMAKE_CXX_STANDARD 0
#endif

#if defined(_MSVC_LANG)
    #define CSV_COMPILER_CXX_LEVEL _MSVC_LANG
#else
    #define CSV_COMPILER_CXX_LEVEL __cplusplus
#endif

#if CSV_CMAKE_CXX_STANDARD >= 20 || CSV_COMPILER_CXX_LEVEL >= 202002L
    #define CSV_HAS_CXX20
#endif

#if CSV_CMAKE_CXX_STANDARD >= 17 || CSV_COMPILER_CXX_LEVEL >= 201703L
    #define CSV_HAS_CXX17
#endif

#if CSV_CMAKE_CXX_STANDARD >= 14 || CSV_COMPILER_CXX_LEVEL >= 201402L
    #define CSV_HAS_CXX14
#endif

In the example above, I also included this fix as a test, when you include the csv-fixed.hpp instead of the default csv.hpp it works in all compilers without the /Zc:__cplusplus flag.

Feel free to adapt the solution suggestion!

Thank you!

[gsantner]

Can confirm recent releases don't compile - MSVC with C++23 activated:

[vincentlaucsb]

For now, set /Zc:__cplusplus as a workaround. I will be rewriting the MSVC C++ version detection to fix this shortly (and it may involve your suggestion).

[vincentlaucsb]

Should be fixed by the #300 which uses _MSVC_LANG or __cplusplus (whichever is greater).

I added a new CI test for MSVC w/o CMake (and MinGW if any readers use that).

Let me know if there's still any issues.

[addy90]

Hey @vincentlaucsb thank you for your quick resolving of this issue!

However, i have a question for the new detection code, as defined from here:

csv-parser/include/internal/common.hpp

Line 76 in 7577473

#if (defined(CMAKE_CXX_STANDARD) && CMAKE_CXX_STANDARD == 20) || CSV_CPLUSPLUS >= 202002L

It tells if CMAKE_CXX_STANDARD standard equals == 20 or the _MSVC_LANG or __cplusplus is larger or equal >=.
Same with 17 but NOT with 14, as can be seen below:

#if (defined(CMAKE_CXX_STANDARD) && CMAKE_CXX_STANDARD == 20) || CSV_CPLUSPLUS >= 202002L
#define CSV_HAS_CXX20
#endif

#if (defined(CMAKE_CXX_STANDARD) && CMAKE_CXX_STANDARD == 17) || CSV_CPLUSPLUS >= 201703L
#define CSV_HAS_CXX17
#endif

#if (defined(CMAKE_CXX_STANDARD) && CMAKE_CXX_STANDARD >= 14) || CSV_CPLUSPLUS >= 201402L
#define CSV_HAS_CXX14
#endif

Shouldn't it be like this instead?

#if (defined(CMAKE_CXX_STANDARD) && CMAKE_CXX_STANDARD >= 20) || CSV_CPLUSPLUS >= 202002L
#define CSV_HAS_CXX20
#endif

#if (defined(CMAKE_CXX_STANDARD) && CMAKE_CXX_STANDARD >= 17) || CSV_CPLUSPLUS >= 201703L
#define CSV_HAS_CXX17
#endif

#if (defined(CMAKE_CXX_STANDARD) && CMAKE_CXX_STANDARD >= 14) || CSV_CPLUSPLUS >= 201402L
#define CSV_HAS_CXX14
#endif

In practice, it doesn't make much difference, because if CMAKE_CXX_STANDARD is defined, so is CSV_CPLUSPLUS and as such, it works nontheless. But, for example, having set CMAKE_CXX_STANDARD to 23 would have to rely on the CSV_CPLUSPLUS to work correctly then. Maybe CMAKE_CXX_STANDARD isn't at all good here and shoule be avoided? Like this?

#if CSV_CPLUSPLUS >= 202002L
#define CSV_HAS_CXX20
#endif

#if CSV_CPLUSPLUS >= 201703L
#define CSV_HAS_CXX17
#endif

#if CSV_CPLUSPLUS >= 201402L
#define CSV_HAS_CXX14
#endif

Just asking for clarification purposes so that the code does what it should do.
Thank you for your help!

[vincentlaucsb]

@addy90 You are 100% correct. Currently, the detection macros work as is for C++23, but only by accident (because we are falling back to CSV_CPLUSPLUS as you pointed out).

I will simplify the detection macros to your suggestion in the next version.