Duplicate case constant is a trick that is guaranteed to work IN ALL C COMPILERS regardless of how each of them reports error. For Visual C++, it is simple:
struct X {
int a,b;
int c[10];
};
int _tmain(int argc, _TCHAR* argv[])
{
int dummy;
switch (dummy) {
case sizeof(X):
case sizeof(X):
break;
}
return 0;
}
Compilation result:
------ Build started: Project: cpptest, Configuration: Debug Win32 ------
cpptest.cpp c:\work\cpptest\cpptest\cpptest.cpp(29): error C2196: case value '48' already used
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========
So sizeof the struct X is 48
EDITED (3jun2020):
For gcc or any other compilers that only print "duplicate case value", I use this trick to narrow down the value:
1) add a case value 1==2 (to represent false)
2) by trial and error, narrow down the value e.g. I try to guess the sizeof(X) is >16:
#include <stdio.h>
typedef struct _X {
int a;
char b[10];
} X;
int main()
{
printf("Hello World");
int dummy=0 ;
switch (dummy) {
case 1==2:
case sizeof( X)>16:
//case 16:
break;
}
return 0;
}
result:
main.c: In function ‘main’:
main.c:14:5: error: duplicate case value
case sizeof( X)>16:
^~~~
main.c:13:5: error: previously used here
case 1==2:
so it is false, i.e. sizeof(X)<=16.
3) repeat with some other sensible values. e.g. try to guess that it is 16 i.e. sizeof(X)==16. If it doesn't complain about duplicate case value. Then the expression is true.
4) optionally add a case 16 to verify it e.g.
#include <stdio.h>
typedef struct _X {
int a;
char b[10];
} X;
int main()
{
printf("Hello World");
int dummy=0 ;
switch (dummy) {
// case 1==2:
case sizeof( X):
case 16:
break;
}
return 0;
}
result
main.c: In function ‘main’:
main.c:15:5: error: duplicate case value
case 16:
^~~~
main.c:14:5: error: previously used here
case sizeof( X):
confirming that sizeof(X) is 16.
Alternatively, it is observed that gcc can report multiple duplicates, so this trick is possible for making multiple guesses on a single pass:
#include <stdio.h>
typedef struct _X {
int a;
char b[10];
} X;
int main()
{
printf("Hello World");
int dummy=0 ;
switch (dummy) {
case 1==2: //represents false
case 1==1: //represents true
case sizeof( X)>10:
case sizeof( X)>12:
case sizeof( X)>14:
case sizeof( X)>16:
case sizeof( X)==16:
//case 16:
break;
}
return 0;
}
result
main.c: In function ‘main’:
main.c:14:5: error: duplicate case value
case sizeof( X)>10:
^~~~
main.c:13:5: error: previously used here
case 1==1:
^~~~
main.c:15:5: error: duplicate case value
case sizeof( X)>12:
^~~~
main.c:13:5: error: previously used here
case 1==1:
^~~~
main.c:16:5: error: duplicate case value
case sizeof( X)>14:
^~~~
main.c:13:5: error: previously used here
case 1==1:
^~~~
main.c:17:5: error: duplicate case value
case sizeof( X)>16:
^~~~
main.c:12:5: error: previously used here
case 1==2:
^~~~
main.c:18:5: error: duplicate case value
case sizeof( X)==16:
^~~~
main.c:13:5: error: previously used here
case 1==1:
^~~~
suggesting the sizeof(X) is >10, >12, >14 but is not >16. The ==16 is added as a final guess.
