问题
I have been looking through some MFC code and i came across this expression. It was in OnInitDialog() function, didn't look like it's MFC specific. The variables had some name, value was 0.
int volatile something, somethingElse; //this was global
something, somethingElse = 0; //this was inside the function
Does this make any sense in C++? I know how the comma operator works, though in a free form like here it should be separating expressions. Is a variable name also an expression? This code does compile, so how does this work?
回答1:
This is likely an error in the program. The statement
a, b = c;
Is completely equivalent to
b = c;
Since the comma operator evaluates from left to right and discards all values except the last. Since the expression a has no side effects, it's essentially a no-op.
I would suspect that this is either programmer error or an incorrect translation of code from a different language into C++. You should contact the author to let them know about this.
Hope this helps!
回答2:
Legal but questionable. The part before the comma doesn't do anything at all.
回答3:
something, somethingElse = 0;
probably, it is done to avoid the unused variable warning on variable something an to initialize the somethingElse variable to 0.
回答4:
Does this make any sense in C++?
Yes syntactically it does, but without comments you may not know the developers intentions were (if any) other than maybe suppressing a variable warning.
Is a variable name also an expression?
Yes a variable itself is an expression. Ex. if(<expression>) if(something)
This code does compile, so how does this work?
It works by using the comma operator and ignoring the result of something then assigning 0 to somethingElse. Although something was marked volatile the original developer may of had a compiler that still complained about unused variables and being the clever developer he or she was then decided to suppress with that syntax.
来源：https://stackoverflow.com/questions/9101999/expression-variable-variable-value