c++ - 编译时检查和运行时检查 'at the same time'

Question

假设我有以下简化程序:

Link to godbolt.org :

#include <cassert>

struct Dimensions {

    Dimensions& operator=(int i) {
      assert(i != 0);
      return *this;
    }

};

int getDim();

int main() {
    Dimensions dims;
    dims = getDim();//ok, just use runtime assert
    dims = 0;//compile error wanted here
    return 0;
}

在第一种情况 (getDim) 中，无法检查编译时，因此我们很乐意在运行时检查它。

但是是否也可以在编译时进行检测(对于第二种情况，dims = 0;)，当理论上看起来这是可能的？ (也许有某种重载或包装器？)

Answer 1

使用 gcc 编译器在 C 中执行的典型方法，也适用于 C++:您使用 __builtin_constant_p内置检查表达式是否是持续评估的，然后检查表达式，然后调用用 __attribute__((__warning__)) 声明的函数或与 __attribute__((__error__)) 。就像这样:

#include <cassert>
#include <type_traits>

#define CONCAT(a, b) a ## b
#define XCONCAT(a, b) CONCAT(a, b)
#define maybe_static_maybe_not_assert(expr) do { \
    if (__builtin_constant_p(expr)) { \
            if (!(expr)) { \
                extern __attribute__((__warning__( \
                "static_assert: expression: " #expr " will fail on runtime!" \
                ))) void XCONCAT(maybe_static_maybe_not_assert_warn, __LINE__)(); \
              XCONCAT(maybe_static_maybe_not_assert_warn, __LINE__)(); \
         } \
    } \
    assert(expr); \
} while(0)

struct Dimensions {

    Dimensions& operator=(int i) {
        maybe_static_maybe_not_assert(i != 0);
        return *this;
    }

};

int getDim();

int main() {
    Dimensions dims;
    dims = getDim();
    dims = 0;
    dims = 1;
    return 0;
}

在优化编译时应该发出警告:

In member function 'Dimensions& Dimensions::operator=(int)',
    inlined from 'int main()' at <source>:32:12:
<source>:12:70: warning: call to 'maybe_static_maybe_not_assert_warn21' declared with attribute warning: static_assert: expression: i != 0 will fail on runtime! [-Wattribute-warning]
   12 |                 XCONCAT(maybe_static_maybe_not_assert_warn, __LINE__)(); \
      |                                                                      ^
<source>:21:9: note: in expansion of macro 'maybe_static_maybe_not_assert'
   21 |         maybe_static_maybe_not_assert(i != 0);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Compiler returned: 0

就是这样_FORTIFY_SOURCE在 glibc 中实现.