问题
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?
回答1:
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.
回答2:
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.
回答3:
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.
回答4:
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
回答5:
Simplest C/C++ any compiler/standard compliant solution, was mentioned by @leftaroundabout in the comments above:
int x = *(int*)"abcd";
Or a bit more specific:
int x = *(int32_t*)"abcd";
One more solution, also compliant with any C/C++ compiler/standard (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 "abcd" string literal is casted into array, then anonymous union is used to give a nice symbol-name to the desired numeric result.
来源：https://stackoverflow.com/questions/7755202/multi-character-constant-warnings