time checking of constant function inputs

I want to use gcc to do some compile-time checking on function inputs if the compiler knows that they are constants.

I have a solution that very almost works, and as far as I can see, it should work.

Note: __builtin_constant_p(expression) is supposed to returns whether an expression is known to be a constant at compile time.

Assuming we want to check whether port<2 when calling uart(port), the following code should work:

#include <stdio.h>

void _uart(int port) {
 printf("port is %d", port);
}

#define uart(port) 
 static_assert(__builtin_constant_p(port)? port<2: 1, "parameter port must be < 2"); 
 _uart(port)

int main(void) {
 int x=1;
 uart(x);
}

This works when calling uart(). Unfortunately, it doesn't quite work for non-constant x. For some reason static_assert can't handle the case where x is not a constant, even though in theory __builtin_constant_p() won't even pass it a constant. The error message I get is:

c:>gcc a.cpp -std=c++0x -Os
a.cpp: In function 'int main()':
a.cpp:13: error: 'x' cannot appear in a constant-expression

Any ideas?

---

Your code works with g++ (GCC) 4.8.2.

- but not with optimization, as you correctly noted.

If only we could use

static_assert(__builtin_choose_expr(__builtin_constant_p(port), 
                          port<2, 1), "parameter port must be < 2")

- but unfortunately the __builtin_choose_expr construct is currently only available for C.

However, there is a C++ patch which sadly didn't make it into the release yet.

---

You can try the trick used in the Linux kernel:

What is ":-!!" in C code?

The (somewhat horrible) Linux kernel macro is less strict about what kinds of expressions are allowed in the parameter.

链接地址: http://www.djcxy.com/p/73564.html

上一篇: 编译器是否优化了大小为0的结构？

下一篇: 实时检查恒定功能输入