Multi-character constant warnings
C++CCastingCompiler WarningsPortabilityC++ Problem Overview
Why is this a warning? I think there are many cases when is more clear to use multi-char int constants instead of "no meaning" numbers or instead of defining const variables with same value. When parsing wave/tiff/other file types is more clear to compare the read values with some 'EVAW', 'data', etc instead of their corresponding values.
Sample code:
int waveHeader = 'EVAW';
Why does this give a warning?
C++ Solutions
Solution 1 - C++
According to the standard (§6.4.4.4/10)
> The value of an integer character constant containing more than one > character (e.g., 'ab'), [...] is implementation-defined.
long x = '\xde\xad\xbe\xef'; // yes, single quotes
This is valid ISO 9899:2011 C. It compiles without warning under gcc
with -Wall
, and a “multi-character character constant” warning with -pedantic
.
From Wikipedia:
> Multi-character constants (e.g. 'xy') are valid, although rarely > useful — they let one store several characters in an integer (e.g. 4 > ASCII characters can fit in a 32-bit integer, 8 in a 64-bit one). > Since the order in which the characters are packed into one int is not > specified, portable use of multi-character constants is difficult.
For portability sake, don't use multi-character constants with integral types.
Solution 2 - C++
This warning is useful for programmers that would mistakenly write 'test'
where they should have written "test"
.
This happen much more often than programmers that do actually want multi-char int constants.
Solution 3 - C++
If you're happy you know what you're doing and can accept the portability problems, on GCC for example you can disable the warning on the command line:
-Wno-multichar
I use this for my own apps to work with AVI and MP4 file headers for similar reasons to you.
Solution 4 - C++
Even if you're willing to look up what behavior your implementation defines, multi-character constants will still vary with endianness.
Better to use a (POD) struct { char[4] }; ... and then use a UDL like "WAVE"_4cc to easily construct instances of that class
Solution 5 - C++
Simplest C/C++ any compiler/standard compliant solution, was mentioned by @leftaroundabout in comments above:
int x = *(int*)"abcd";
Or a bit more specific:
int x = *(int32_t*)"abcd";
One more solution, also compliant with C/C++ compiler/standard since C99 (except clang++, which has a known bug):
int x = ((union {char s[5]; int number;}){"abcd"}).number;
/* just a demo check: */
printf("x=%d stored %s byte first\n", x, x==0x61626364 ? "MSB":"LSB");
Here anonymous union is used to give a nice symbol-name to the desired numeric result, "abcd" string is used to initialize the lvalue of compound literal (C99).
Solution 6 - C++
If you want to disable this warning it is important to know that there are two related warning parameters in GCC and Clang: https://stackoverflow.com/q/47304722/4915707