Anyone know why this C code won't compile?

#include <stdio.h>
int const NAMESIZE = 40;
int const ADDRSIZE = 80;
typedef char NameType[NAMESIZE];
typedef char Addr开发者_高级运维Type[ADDRSIZE];

typedef struct
{
    NameType name;
    AddrType address;
    double salary;
    unsigned int id;
}EmpRecType;

int main(int * argc, char * argv[])
{
    EmpRecType employee;
    return 0;
}

If I use #define instead of const it compiles. this is the error:

employee.c:5:14: error: variably modified 'NameType' at file scope employee.c:6:14: error: variably modified 'AddrType' at file scope

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One of the differences between C and C++ is that in C++ a const int object is a constant, i.e. it can be used to form constant expressions. In C, on the other hand, a const int object is not a constant at all (it's more like "unchangeable variable").

Meanwhile, array size for file scope arrays in C is required to be a constant expression, which is why your const int object does not work in that role. (The above means, BTW, that your code will compile perfectly fine as C++, but won't compile as C.)

In C language to define named constants you have to use either #define or enums. In your specific case it could be done as follows

#define NAMESIZE 40
#define ADDRSIZE 80

P.S. If you replace your file-scope arrays with local arrays, your C code will compile as is, i.e. with const int objects as array sizes, because modern C (ANSI C99) supports variable-length arrays (VLA) in local scope. (And your arrays will be VLA in that case). In older versions of C (like ANSI C89/90) the code will not compile even with local arrays.

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Those const declarations, in C, just define some read only memory, they are not true constants. They can't be evaluated until runtime which is too late for the array declarations.