I wanted to do a program with concatenating strings 'strcat()', where this function accepts two strings, but the second must be a constant string. This is a problem, because if I need more than 1 concatenating, then I need more than 1 constant strings, instead of using the same every time.
I was just wondering why is it necessary that the second string is constant? I know it's because C language is just made like that, but I was wondering why.
When there is the "constant" keyword in front of a parameter it means that the parameter doesn't change value inside the function. Which can be true foa strcat() function.
What is the benefit? The compiler can optimize by replacing directly the value. For example
When calling boo the parameter will be copied in a temporarily variable, my. My changes its value, there is no other way (it bocomes "hey").Code:char* foo(const char* my) { return my; } char* boo(char* my) { my = "hey"; return my; }
When calling foo, my doesn't change. So the compiler can optimize and not create a temporarily varialbe my. Just replace my with the value inserted.
Not sure how much it has to do with optimization. It just might interest the caller whether a function will modify its pointer arguments or not.
That const tells you whether you can pass a string literal (which are unmodifiable) as the second argument. Unfortunately, C allows implicit conversion of const char* to char*, so you could also pass a string literal to an argument that is going to be modified in the function (and probably crash the program doing so), but at least compilers emit warnings if you do that (at least GCC compiling as C++ does).
Code:#include <ctype.h> void foo(char* str) { str[0] = toupper(str[0]); } void bar(const char* str) { //str[0] = toupper(str[0]); } int main() { foo("hello"); //wrong: function is going to attempt to modify a literal bar("hello"); //safe: function won't modify argument return 0; }That doesn't compile (you are trying to return a non-constant pointer from a constant pointer).What is the benefit? The compiler can optimize by replacing directly the value. For example
And even then, what is constant is the string pointed to by my. What is not constant is the copy of the pointer itself:
Code:const char* foo(const char* my) { my = "hey"; //my now points to string literal "hey" return my; }
Last edited by anon; 11-29-2009 at 10:27 AM.
I might be wrong.
Quoted more than 1000 times (I hope).Thank you, anon. You sure know how to recognize different types of trees from quite a long way away.
