这个例子应该如何工作？ 它对我来说是错误的

Question

I found this example at https://en.cppreference.com/w/c/string/byte/strcpy

#define __STDC_WANT_LIB_EXT1__ 1
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
 
int main(void)
{
    char *src = "Take the test.";
//  src[0] = 'M' ; // this would be undefined behavior
    char dst[strlen(src) + 1]; // +1 to accomodate for the null terminator
    strcpy(dst, src);
    dst[0] = 'M'; // OK
    printf("src = %s\ndst = %s\n", src, dst);
 
#ifdef __STDC_LIB_EXT1__
    set_constraint_handler_s(ignore_handler_s);
    int r = strcpy_s(dst, sizeof dst, src);
    printf("dst = \"%s\", r = %d\n", dst, r);
    r = strcpy_s(dst, sizeof dst, "Take even more tests.");
    printf("dst = \"%s\", r = %d\n", dst, r);
#endif
}

Using nvcc compiler, I get an error on the line char dst[strlen(src) + 1];"expression must have a constant value."

Using Visual C++, I get that error and another error, "C++ a value of type cannot be used to initialize an entity of type," on the line char *src = "Take the test.";

If I compile and run it on the site, it's fine.

Answer 1

This feature is called "flexible array member" or in some cases "variable length arrays". It is supported in 'c' starting with 'c99' standard. gcc and microsoft 'c' supoorted it before the standard as well.

c++ in general does not support this feature. Some support was added in c++14 requireing simple expressions as array indexes.

So, you might be able to compile it with a 'c' compiler which does support c99 or later standards. Some specific qualifiers could be needed at the compilation command line as well.

The easiest way to work around it in a portable way would be using dynamic array allocation with malloc:

 char *dst = malloc(strlen(src) + 1);
 strcpy(dst, src);

and later use strlen(dst) instead of the sizeof dst ;