结构堆栈错误：在“=”标记之前应为“:”、“、”、“;”、“}”或“__attribute__”

Question

Code (test.c):

#include <stdio.h>
struct stack {
    double a[100];
    int maxSize = 100;
};
int main() {
    return 0;
}

Compile command:

gcc -Wall -o "test" "test.c"

Errors:

gcc -Wall -o "test" "test.c" (in directory: /home/amey)
test.c:4:17: error: expected ‘:’, ‘,’, ‘;’, ‘}’ or ‘__attribute__’ before ‘=’ token
     int maxSize = 100;
                 ^
Compilation failed.

Why is this not compiling?

Answer 1

You cannot declare default values in a struct declaration in C. This is simply not allowed in the C language.

What you can do is this:

...
int main()
{
  struct stack mystack;
  mystack.maxSize = 100;
  ...

or

...
int main()
{
  struct stack mystack = {.maxSize = 100};
  ...

But anyway it doesn't make much sense to have a variable maxSize because the size of a is fixed to 100 anyway. The proper way would be a #define MAXSIZE 100 or const int MAXSIZE = 1000; , and then declare double a[MAXSIZE]; and drop the field maxSize alltogether.

That being said, you probably want something like this:

#include <stdio.h>
 
#define MAXSTACKSIZE 100
// or 
// const int MAXSIZE = 100;
 
struct stack {
    double stack[MAXSTACKSIZE];
    int stackpointer;
};
 
void InitializeStack(struct stack *thestack)
{
  thestack->stackpointer = 0;
}
 
void push(struct stack *thestack, double value)
{
   // to be written (3-4 lines including error checking)
}
 
double pop(struct stack *thestack)
{
   // to be written (3-4 lines including error checking)
}
 
int main() {
    struct stack mystack;
    InitializeStack(&mystack);
    push(&mystack, 1.234);
    printf("%f\n", pop(&mystack));   // output should be 1.234
    return 0;
}