是否有可能编写一个模板，根据某个成员函数是否定义在类上而改变行为?
下面是我想写的一个简单的例子:
template<class T>
std::string optionalToString(T* obj)
{
if (FUNCTION_EXISTS(T->toString))
return obj->toString();
else
return "toString not defined";
}
因此，如果类T定义了toString()，那么它就使用它;否则，它就不会。我不知道如何做的神奇部分是“FUNCTION_EXISTS”部分。
当前回答
用c++ 20你可以写以下代码:
template<typename T>
concept has_toString = requires(const T& t) {
t.toString();
};
template<typename T>
std::string optionalToString(const T& obj)
{
if constexpr (has_toString<T>)
return obj.toString();
else
return "toString not defined";
}
其他回答
c++允许SFINAE用于此(注意，在c++ 11特性中，这更简单，因为它支持在几乎任意表达式上扩展SFINAE -下面的代码是为使用常见的c++ 03编译器而设计的):
#define HAS_MEM_FUNC(func, name) \
template<typename T, typename Sign> \
struct name { \
typedef char yes[1]; \
typedef char no [2]; \
template <typename U, U> struct type_check; \
template <typename _1> static yes &chk(type_check<Sign, &_1::func > *); \
template <typename > static no &chk(...); \
static bool const value = sizeof(chk<T>(0)) == sizeof(yes); \
}
上面的模板和宏尝试实例化一个模板，给它一个成员函数指针类型，以及实际的成员函数指针。如果类型不匹配，SFINAE会导致模板被忽略。用法:
HAS_MEM_FUNC(toString, has_to_string);
template<typename T> void
doSomething() {
if(has_to_string<T, std::string(T::*)()>::value) {
...
} else {
...
}
}
但是注意，你不能在if分支中调用toString函数。由于编译器将在两个分支中检查有效性，因此在函数不存在的情况下会失败。一种方法是再次使用SFINAE (enable_if也可以从boost中获得):
template<bool C, typename T = void>
struct enable_if {
typedef T type;
};
template<typename T>
struct enable_if<false, T> { };
HAS_MEM_FUNC(toString, has_to_string);
template<typename T>
typename enable_if<has_to_string<T,
std::string(T::*)()>::value, std::string>::type
doSomething(T * t) {
/* something when T has toString ... */
return t->toString();
}
template<typename T>
typename enable_if<!has_to_string<T,
std::string(T::*)()>::value, std::string>::type
doSomething(T * t) {
/* something when T doesnt have toString ... */
return "T::toString() does not exist.";
}
享受使用它的乐趣。它的优点是它也适用于重载的成员函数，也适用于const成员函数(记得使用std::string(T::*)() const作为成员函数指针类型!)
下面是一些用法片段: *所有这些的核心都在更深处
在给定的类中检查成员x。可以是var, func, class, union或enum:
CREATE_MEMBER_CHECK(x);
bool has_x = has_member_x<class_to_check_for_x>::value;
检查成员函数void x():
//Func signature MUST have T as template variable here... simpler this way :\
CREATE_MEMBER_FUNC_SIG_CHECK(x, void (T::*)(), void__x);
bool has_func_sig_void__x = has_member_func_void__x<class_to_check_for_x>::value;
检查成员变量x:
CREATE_MEMBER_VAR_CHECK(x);
bool has_var_x = has_member_var_x<class_to_check_for_x>::value;
检查成员类x:
CREATE_MEMBER_CLASS_CHECK(x);
bool has_class_x = has_member_class_x<class_to_check_for_x>::value;
检查成员工会x:
CREATE_MEMBER_UNION_CHECK(x);
bool has_union_x = has_member_union_x<class_to_check_for_x>::value;
检查成员enum x:
CREATE_MEMBER_ENUM_CHECK(x);
bool has_enum_x = has_member_enum_x<class_to_check_for_x>::value;
检查任何成员函数x，而不考虑签名:
CREATE_MEMBER_CHECK(x);
CREATE_MEMBER_VAR_CHECK(x);
CREATE_MEMBER_CLASS_CHECK(x);
CREATE_MEMBER_UNION_CHECK(x);
CREATE_MEMBER_ENUM_CHECK(x);
CREATE_MEMBER_FUNC_CHECK(x);
bool has_any_func_x = has_member_func_x<class_to_check_for_x>::value;
OR
CREATE_MEMBER_CHECKS(x); //Just stamps out the same macro calls as above.
bool has_any_func_x = has_member_func_x<class_to_check_for_x>::value;
细节和核心:
/*
- Multiple inheritance forces ambiguity of member names.
- SFINAE is used to make aliases to member names.
- Expression SFINAE is used in just one generic has_member that can accept
any alias we pass it.
*/
//Variadic to force ambiguity of class members. C++11 and up.
template <typename... Args> struct ambiguate : public Args... {};
//Non-variadic version of the line above.
//template <typename A, typename B> struct ambiguate : public A, public B {};
template<typename A, typename = void>
struct got_type : std::false_type {};
template<typename A>
struct got_type<A> : std::true_type {
typedef A type;
};
template<typename T, T>
struct sig_check : std::true_type {};
template<typename Alias, typename AmbiguitySeed>
struct has_member {
template<typename C> static char ((&f(decltype(&C::value))))[1];
template<typename C> static char ((&f(...)))[2];
//Make sure the member name is consistently spelled the same.
static_assert(
(sizeof(f<AmbiguitySeed>(0)) == 1)
, "Member name specified in AmbiguitySeed is different from member name specified in Alias, or wrong Alias/AmbiguitySeed has been specified."
);
static bool const value = sizeof(f<Alias>(0)) == 2;
};
宏(El Diablo!):
CREATE_MEMBER_CHECK:
//Check for any member with given name, whether var, func, class, union, enum.
#define CREATE_MEMBER_CHECK(member) \
\
template<typename T, typename = std::true_type> \
struct Alias_##member; \
\
template<typename T> \
struct Alias_##member < \
T, std::integral_constant<bool, got_type<decltype(&T::member)>::value> \
> { static const decltype(&T::member) value; }; \
\
struct AmbiguitySeed_##member { char member; }; \
\
template<typename T> \
struct has_member_##member { \
static const bool value \
= has_member< \
Alias_##member<ambiguate<T, AmbiguitySeed_##member>> \
, Alias_##member<AmbiguitySeed_##member> \
>::value \
; \
}
CREATE_MEMBER_VAR_CHECK:
//Check for member variable with given name.
#define CREATE_MEMBER_VAR_CHECK(var_name) \
\
template<typename T, typename = std::true_type> \
struct has_member_var_##var_name : std::false_type {}; \
\
template<typename T> \
struct has_member_var_##var_name< \
T \
, std::integral_constant< \
bool \
, !std::is_member_function_pointer<decltype(&T::var_name)>::value \
> \
> : std::true_type {}
CREATE_MEMBER_FUNC_SIG_CHECK:
//Check for member function with given name AND signature.
#define CREATE_MEMBER_FUNC_SIG_CHECK(func_name, func_sig, templ_postfix) \
\
template<typename T, typename = std::true_type> \
struct has_member_func_##templ_postfix : std::false_type {}; \
\
template<typename T> \
struct has_member_func_##templ_postfix< \
T, std::integral_constant< \
bool \
, sig_check<func_sig, &T::func_name>::value \
> \
> : std::true_type {}
CREATE_MEMBER_CLASS_CHECK:
//Check for member class with given name.
#define CREATE_MEMBER_CLASS_CHECK(class_name) \
\
template<typename T, typename = std::true_type> \
struct has_member_class_##class_name : std::false_type {}; \
\
template<typename T> \
struct has_member_class_##class_name< \
T \
, std::integral_constant< \
bool \
, std::is_class< \
typename got_type<typename T::class_name>::type \
>::value \
> \
> : std::true_type {}
CREATE_MEMBER_UNION_CHECK:
//Check for member union with given name.
#define CREATE_MEMBER_UNION_CHECK(union_name) \
\
template<typename T, typename = std::true_type> \
struct has_member_union_##union_name : std::false_type {}; \
\
template<typename T> \
struct has_member_union_##union_name< \
T \
, std::integral_constant< \
bool \
, std::is_union< \
typename got_type<typename T::union_name>::type \
>::value \
> \
> : std::true_type {}
CREATE_MEMBER_ENUM_CHECK:
//Check for member enum with given name.
#define CREATE_MEMBER_ENUM_CHECK(enum_name) \
\
template<typename T, typename = std::true_type> \
struct has_member_enum_##enum_name : std::false_type {}; \
\
template<typename T> \
struct has_member_enum_##enum_name< \
T \
, std::integral_constant< \
bool \
, std::is_enum< \
typename got_type<typename T::enum_name>::type \
>::value \
> \
> : std::true_type {}
CREATE_MEMBER_FUNC_CHECK:
//Check for function with given name, any signature.
#define CREATE_MEMBER_FUNC_CHECK(func) \
template<typename T> \
struct has_member_func_##func { \
static const bool value \
= has_member_##func<T>::value \
&& !has_member_var_##func<T>::value \
&& !has_member_class_##func<T>::value \
&& !has_member_union_##func<T>::value \
&& !has_member_enum_##func<T>::value \
; \
}
CREATE_MEMBER_CHECKS:
//Create all the checks for one member. Does NOT include func sig checks.
#define CREATE_MEMBER_CHECKS(member) \
CREATE_MEMBER_CHECK(member); \
CREATE_MEMBER_VAR_CHECK(member); \
CREATE_MEMBER_CLASS_CHECK(member); \
CREATE_MEMBER_UNION_CHECK(member); \
CREATE_MEMBER_ENUM_CHECK(member); \
CREATE_MEMBER_FUNC_CHECK(member)
这是c++ 17中另一种实现方法(灵感来自boost:hana)。
该解决方案不需要has_something<T> SFINAE类型trait类。
解决方案
////////////////////////////////////////////
// has_member implementation
////////////////////////////////////////////
#include <type_traits>
template<typename T, typename F>
constexpr auto has_member_impl(F&& f) -> decltype(f(std::declval<T>()), true)
{
return true;
}
template<typename>
constexpr bool has_member_impl(...) { return false; }
#define has_member(T, EXPR) \
has_member_impl<T>( [](auto&& obj)->decltype(obj.EXPR){} )
Test
////////////////////////////////////////////
// Test
////////////////////////////////////////////
#include <iostream>
#include <string>
struct Example {
int Foo;
void Bar() {}
std::string toString() { return "Hello from Example::toString()!"; }
};
struct Example2 {
int X;
};
template<class T>
std::string optionalToString(T* obj)
{
if constexpr(has_member(T, toString()))
return obj->toString();
else
return "toString not defined";
}
int main() {
static_assert(has_member(Example, Foo),
"Example class must have Foo member");
static_assert(has_member(Example, Bar()),
"Example class must have Bar() member function");
static_assert(!has_member(Example, ZFoo),
"Example class must not have ZFoo member.");
static_assert(!has_member(Example, ZBar()),
"Example class must not have ZBar() member function");
Example e1;
Example2 e2;
std::cout << "e1: " << optionalToString(&e1) << "\n";
std::cout << "e1: " << optionalToString(&e2) << "\n";
}
我在另一个线程中对此写了一个答案(与上面的解决方案不同)，也检查继承的成员函数:
SFINAE检查继承的成员函数
以下是该解决方案的一些例子:
例二:
我们正在检查一个具有以下签名的成员: T::const_iterator begin(
template<class T> struct has_const_begin
{
typedef char (&Yes)[1];
typedef char (&No)[2];
template<class U>
static Yes test(U const * data,
typename std::enable_if<std::is_same<
typename U::const_iterator,
decltype(data->begin())
>::value>::type * = 0);
static No test(...);
static const bool value = sizeof(Yes) == sizeof(has_const_begin::test((typename std::remove_reference<T>::type*)0));
};
请注意，它甚至检查方法的常量，并且也适用于基本类型。(我的意思是has_const_begin<int>::value为false，不会导致编译时错误。)
示例2
现在我们正在寻找签名:void foo(MyClass&， unsigned)
template<class T> struct has_foo
{
typedef char (&Yes)[1];
typedef char (&No)[2];
template<class U>
static Yes test(U * data, MyClass* arg1 = 0,
typename std::enable_if<std::is_void<
decltype(data->foo(*arg1, 1u))
>::value>::type * = 0);
static No test(...);
static const bool value = sizeof(Yes) == sizeof(has_foo::test((typename std::remove_reference<T>::type*)0));
};
请注意，MyClass不一定是默认可构造的或满足任何特殊的概念。该技术也适用于模板成员。
我急切地等待有关这方面的意见。
template<class T>
auto optionalToString(T* obj)
->decltype( obj->toString(), std::string() )
{
return obj->toString();
}
template<class T>
auto optionalToString(T* obj)
->decltype( std::string() )
{
throw "Error!";
}
