Static array in function

[sandromani]

Hi,
just wondering, why does it work when I create a static array with size unknown at compile time inside a function? I.e. small example:

#include <iostream>
 
void cpy(int a[], unsigned n){
    int b[n];
    std::copy(a, a+n, b);
    std::cout<<b[0]<<" "<<b[1]<<" "<<b[2]<<" "<<b[3]<<" "<<b[4]<<"\n";
}
 
int main(int argc, char **argv){
    unsigned n;
    std::cin>>n;
    int a[5]={1,2,3,4,5};
    cpy(a, n);
    return 0;
}
 

The code runs fine, the compiler (gcc 4.5.1 / icpc 11.1) does not complain (though if I change int a[5] to int a[n] in main, the code does not compile as expected).
Is it possibly legal because n is known when local storage is allocated on the stack when the function is called? (Though if I add std::cin>>n; before int b[n] the code still compiles, which is surprising).

Thanks

[horace1]

in your case

void cpy(int a[], unsigned n){
    int b[n];
    std::copy(a, a+n, b);
    std::cout<<b[0]<<" "<<b[1]<<" "<<b[2]<<" "<<b[3]<<" "<<b[4]<<"\n";
}
 

b[] is an automactic local array with memory being allocated at run time on the stack
if it was static, e.g.

void cpy(int a[], unsigned n){
    static int b[n];
    std::copy(a, a+n, b);
    std::cout<<b[0]<<" "<<b[1]<<" "<<b[2]<<" "<<b[3]<<" "<<b[4]<<"\n";
}
 

memory for static variables is allocated by the compiler at compile time and hence the compiler has to know the size of the array to allocate.
with gcc the above code gives

xxx.cpp||In function 'void cpy(int*, unsigned int)':|
xxx.cpp|4|error: storage size of 'b' isn't constant|
||=== Build finished: 1 errors, 0 warnings ===|
 

[sandromani]

Ok, makes sense, thanks for the explanation!