我知道 this question其中提到了 Boost 的“STATIC WARNING”,但我想再问一次,具体来说,我如何实现一个 static_warning
,它的操作类似于 static_assert
但只发出一个 在编译时发出警告,而不是中止编译错误。
我想要类似于 Alexandrescu 在 C++11 之前提出的静态断言提案,它以某种方式设法打印一些有用的上下文信息作为错误的一部分。
要求用户启用某些标准编译器警告以使此构造工作(可能是“无效指针转换”或“违反严格的别名规则”)是可以接受的——任何应该属于正常编译器的警告编译反正可以用。
简而言之,我希望 static_warning(false, "Hello world");
创建一个编译器警告,它应该以某种方式在警告消息中包含字符串“hello world”。在 GCC 和 MSVC 中,这可能吗?如何实现?
我很乐意为任何特别聪明的解决方案提供小额奖励。
作为一点解释:我在考虑 this question 时得到了这个想法。 :静态警告将是跟踪复杂模板特化的编译时过程的有用方法,否则很难调试。静态警告可以用作编译器发出“我现在正在编译这部分代码”的简单信标。
更新。理想情况下,将在以下设置中触发警告:
template <typename T> struct Foo
{
static_warning(std::is_pointer<T>::value, "Attempting to use pointer type.");
// ...
};
int main() { Foo<int> a; Foo<int*> b; }
最佳答案
利用 Michael E 的评论:
#if defined(__GNUC__)
#define DEPRECATE(foo, msg) foo __attribute__((deprecated(msg)))
#elif defined(_MSC_VER)
#define DEPRECATE(foo, msg) __declspec(deprecated(msg)) foo
#else
#error This compiler is not supported
#endif
#define PP_CAT(x,y) PP_CAT1(x,y)
#define PP_CAT1(x,y) x##y
namespace detail
{
struct true_type {};
struct false_type {};
template <int test> struct converter : public true_type {};
template <> struct converter<0> : public false_type {};
}
#define STATIC_WARNING(cond, msg) \
struct PP_CAT(static_warning,__LINE__) { \
DEPRECATE(void _(::detail::false_type const& ),msg) {}; \
void _(::detail::true_type const& ) {}; \
PP_CAT(static_warning,__LINE__)() {_(::detail::converter<(cond)>());} \
}
// Note: using STATIC_WARNING_TEMPLATE changes the meaning of a program in a small way.
// It introduces a member/variable declaration. This means at least one byte of space
// in each structure/class instantiation. STATIC_WARNING should be preferred in any
// non-template situation.
// 'token' must be a program-wide unique identifier.
#define STATIC_WARNING_TEMPLATE(token, cond, msg) \
STATIC_WARNING(cond, msg) PP_CAT(PP_CAT(_localvar_, token),__LINE__)
宏可以在命名空间、结构和函数范围内调用。给定输入:
#line 1
STATIC_WARNING(1==2, "Failed with 1 and 2");
STATIC_WARNING(1<2, "Succeeded with 1 and 2");
struct Foo
{
STATIC_WARNING(2==3, "2 and 3: oops");
STATIC_WARNING(2<3, "2 and 3 worked");
};
void func()
{
STATIC_WARNING(3==4, "Not so good on 3 and 4");
STATIC_WARNING(3<4, "3 and 4, check");
}
template <typename T> struct wrap
{
typedef T type;
STATIC_WARNING(4==5, "Bad with 4 and 5");
STATIC_WARNING(4<5, "Good on 4 and 5");
STATIC_WARNING_TEMPLATE(WRAP_WARNING1, 4==5, "A template warning");
};
template struct wrap<int>;
GCC 4.6(在默认警告级别)产生:
static_warning.cpp: In constructor ‘static_warning1::static_warning1()’: static_warning.cpp:1:1: warning: ‘void static_warning1::_(const detail::false_type&)’ is deprecated (declared at static_warning.cpp:1): Failed with 1 and 2 [-Wdeprecated-declarations] static_warning.cpp: In constructor ‘Foo::static_warning6::static_warning6()’: static_warning.cpp:6:3: warning: ‘void Foo::static_warning6::_(const detail::false_type&)’ is deprecated (declared at static_warning.cpp:6): 2 and 3: oops [-Wdeprecated-declarations] static_warning.cpp: In constructor ‘func()::static_warning12::static_warning12()’: static_warning.cpp:12:3: warning: ‘void func()::static_warning12::_(const detail::false_type&)’ is deprecated (declared at static_warning.cpp:12): Not so good on 3 and 4 [-Wdeprecated-declarations] static_warning.cpp: In constructor ‘wrap<T>::static_warning19::static_warning19() [with T = int]’: static_warning.cpp:24:17: instantiated from here static_warning.cpp:19:3: warning: ‘void wrap<T>::static_warning19::_(const detail::false_type&) [with T = int]’ is deprecated (declared at static_warning.cpp:19): Bad with 4 and 5 [-Wdeprecated-declarations]
虽然 Visual C++ 2010(在/W3 或更高版本)说:
warnproj.cpp(1): warning C4996: 'static_warning1::_': Failed with 1 and 2 warnproj.cpp(1) : see declaration of 'static_warning1::_' warnproj.cpp(6): warning C4996: 'Foo::static_warning6::_': 2 and 3: oops warnproj.cpp(6) : see declaration of 'Foo::static_warning6::_' warnproj.cpp(12): warning C4996: 'func::static_warning12::_': Not so good on 3 and 4 warnproj.cpp(12) : see declaration of 'func::static_warning12::_' warnproj.cpp(19): warning C4996: 'wrap<T>::static_warning19::_': Bad with 4 and 5 with [ T=int ] warnproj.cpp(19) : see declaration of 'wrap<T>::static_warning19::_' with [ T=int ] warnproj.cpp(19) : while compiling class template member function 'wrap<T>::static_warning19::static_warning19(void)' with [ T=int ] warnproj.cpp(24) : see reference to class template instantiation 'wrap<T>::static_warning19' being compiled with [ T=int ]
Linux 上的 Clang++ 3.1 产生了可以说更好的输出(颜色未显示):
tst3.cpp:1:1: warning: '_' is deprecated: Failed with 1 and 2 [-Wdeprecated-declarations] STATIC_WARNING(1==2, "Failed with 1 and 2"); ^ tst3.cpp:24:38: note: expanded from macro 'STATIC_WARNING' PP_CAT(static_warning,__LINE__)() {_(::detail::converter<(cond)>());} \ ^ tst3.cpp:6:3: warning: '_' is deprecated: 2 and 3: oops [-Wdeprecated-declarations] STATIC_WARNING(2==3, "2 and 3: oops"); ^ tst3.cpp:24:38: note: expanded from macro 'STATIC_WARNING' PP_CAT(static_warning,__LINE__)() {_(::detail::converter<(cond)>());} \ ^ tst3.cpp:12:3: warning: '_' is deprecated: Not so good on 3 and 4 [-Wdeprecated-declarations] STATIC_WARNING(3==4, "Not so good on 3 and 4"); ^ tst3.cpp:24:38: note: expanded from macro 'STATIC_WARNING' PP_CAT(static_warning,__LINE__)() {_(::detail::converter<(cond)>());} \ ^ tst3.cpp:19:3: warning: '_' is deprecated: Bad with 4 and 5 [-Wdeprecated-declarations] STATIC_WARNING(4==5, "Bad with 4 and 5"); ^ tst3.cpp:24:38: note: expanded from macro 'STATIC_WARNING' PP_CAT(static_warning,__LINE__)() {_(::detail::converter<(cond)>());} \ ^ tst3.cpp:23:17: note: in instantiation of member function 'wrap<int>::static_warning19::static_warning19' requested here template struct wrap<int> ^ 4 warnings generated.
关于c++ - 是否存在 static_warning?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/53648326/