问题
int main( const int argc , const char[] const argv)
As Effective C++ Item#3 states "Use const whenever possible", I start thinking "why not make these 'constant' parameters const
"?.
Is there any scenario in which the value of argc
is modified in a program?
回答1:
In this case, history is a factor. C defined these inputs as "not constant", and compatibility with (a good portion of) existing C code was an early goal of C++.
Some UNIX APIs, such as getopt
, actually do manipulate argv[]
, so it can't be made const
for that reason also.
(Aside: Interestingly, although getopt
's prototype suggests it won't modify argv[]
but may modify the strings pointed to, the Linux man page indicates that getopt
permutes its arguments, and it appears they know they're being naughty. The man page at the Open Group does not mention this permutation.)
Putting const
on argc
and argv
wouldn't buy much, and it would invalidate some old-school programming practices, such as:
// print out all the arguments:
while (--argc)
std::cout << *++argv << std::endl;
I've written such programs in C, and I know I'm not alone. I copied the example from somewhere.
回答2:
The C standard (ISO/IEC 9899:2011) says:
5.1.2.2.1 Program startup
¶1 The function called at program startup is named
main
. The implementation declares no prototype for this function. It shall be defined with a return type of int and with no parameters:int main(void) { /* ... */ }
or with two parameters (referred to here as
argc
andargv
, though any names may be used, as they are local to the function in which they are declared):int main(int argc, char *argv[]) { /* ... */ }
or equivalent;10) or in some other implementation-defined manner.
¶2 If they are declared, the parameters to the
main
function shall obey the following constraints:
- The value of
argc
shall be nonnegative.argv[argc]
shall be a null pointer.- If the value of
argc
is greater than zero, the array membersargv[0]
throughargv[argc-1]
inclusive shall contain pointers to strings, which are given implementation-defined values by the host environment prior to program startup. The intent is to supply to the program information determined prior to program startup from elsewhere in the hosted environment. If the host environment is not capable of supplying strings with letters in both uppercase and lowercase, the implementation shall ensure that the strings are received in lowercase.- If the value of
argc
is greater than zero, the string pointed to byargv[0]
represents the program name;argv[0][0]
shall be the null character if the program name is not available from the host environment. If the value ofargc
is greater than one, the strings pointed to byargv[1]
throughargv[argc-1]
represent the program parameters.- The parameters
argc
andargv
and the strings pointed to by theargv
array shall be modifiable by the program, and retain their last-stored values between program startup and program termination.10) Thus,
int
can be replaced by a typedef name defined asint
, or the type ofargv
can be written aschar **argv
, and so on.
Note the last bullet point. It says that both argc
and argv
should be modifiable. They don't have to be modified, but they may be modified.
回答3:
argc
is not normally a constant because the function signature for main()
pre-dates const
.
Since argc is a stack variable, changing it won't affect anything other than your own command line processing.
You are, of course, free to declare it const
if you want.
回答4:
A top level const
on a formal argument is not part of the function type. You can add it or remove it as you like: it only affects what you can do with the argument in the function implementation.
So for argc
you can add freely add a const
.
But for argv
you can not make the character data const
without thereby changing the function signature. Which means that it's then not one of the standard main
function signatures, and will not have to be recognized as a main
function. So, not a good idea.
A good reason for not using the standard main
arguments in non-toy programs is that in Windows they are not able to represent actual program arguments such as filenames with international characters. That's because in Windows they are by very strong convention encoded as Windows ANSI. In Windows you can implement some more portable argument access facility in terms of the GetCommandLine
API function.
Summing up, nothing prevents you from adding const
to argc
, but the most useful const
-ness on argv
would give you a non-standard main
function, most probably not recognized as such. Happily (in an ironic way) there are good reason to not use the standard main
arguments for portable serious code. Quite simply, for the in-practice they only support old ASCII, with only English alphabet letters.
回答5:
The signature of main
is somewhat of a historical artifact from C
. Historically C
did not have const
.
However, you can declare your parameter const
since the effects of const are compile time only.
回答6:
Because argc
is a local variable (and, in C++, not a reference or something), and because the special place of main
means that backwards compatibility shenanigans grant it a huge amount of leeway with no compelling reason to force applications to make it const.
main() {}
int main() {}
main() { return 0; }
main(int argc, char* argv[]) { return 0; }
int main(const int argc, const char** argv) { /* no return*/ }
these and many other variations will compile on a wide range of C and C++ compilers.
So ultimately it's not that argc isn't const, just that it doesn't have to be, but it can be if you want it to be.
http://ideone.com/FKldHF, C example:
main(const int argc, const char* argv[]) { return 0; }
http://ideone.com/m1qc9c, C++ example
main(const int argc) {}
回答7:
Aside from the historical reasons, a good reason to keep argc and argv non-const
is that the compiler implementation doesn't know what you're going to do with the arguments to main, it just knows that it must give you those arguments.
When you are defining your own functions and associated prototypes, you know which parameters you can make const
and which ones your function will modify.
Taken to an extreme, you could declare that all parameters to all functions should be declared const
, and then if you had a reason to change them (e.g. decrement an index to search through an array), you'd have to make local non-const
variables and copy the const
arguments' values into those variables. That makes for busy-work and extra LOC with no real benefit. A decent static analyzer will pick up if you're not modifying the value of an argument, and recommend that you make the parameter const
.
来源:https://stackoverflow.com/questions/20558418/why-is-argc-not-a-constant