[英]Restrict parameter type in template instantiation
I'm trying to trigger a compile time error if the user of my library tries to instantiate a template with a type that is not appropriate. 如果我的库的用户尝试使用不合适的类型实例化模板,我试图触发编译时错误。 I've implemented:
我实施了:
template <typename T>
struct good_type { enum { value = false }; };
template <>
struct good_type<string> { enum { value = true }; };
template <>
struct good_type<int64_t> { enum { value = true }; };
template <typename T>
struct X
{
BOOST_STATIC_ASSERT(good_type<T>::value);
};
int main(int argc, char** argv)
{
X<string> x1;
X<int64_t> x2;
X<float> x3;
return 0;
}
which works, but the message I get from gcc is a bit surprising: 这是有效的,但我从gcc得到的消息有点令人惊讶:
error: invalid application of 'sizeof' to incomplete type 'boost::STATIC_ASSERTION_FAILURE<false>'
Should I be using a different Boost macro? 我应该使用不同的Boost宏吗? Is there a better way to do this?
有一个更好的方法吗?
Thanks! 谢谢!
You can use boost::enable_if
, along with typelist. 您可以使用
boost::enable_if
以及typelist。
Define a typelist which contains all the types which you want to support, and write some metafunction(s), to check if a given type exists in the list or not, and then pass the value which metafunction returns to enable_if
, so as to enable/disable the class. 定义一个包含所有要支持的类型的类型列表 ,并编写一些元函数,检查列表中是否存在给定类型 ,然后将元函数返回的值传递给
enable_if
,以便启用/禁用该类。
Alright, I wrote a code for demo. 好吧,我写了一个演示代码。 Its not using
boost::enable_if
though (that is for you to experiment with). 它虽然没有使用
boost::enable_if
(你可以boost::enable_if
)。
Here is the framework first: 首先是框架:
////////////////////////////////////////////////////////////
//framework
struct null_type {};
template<typename H, typename T=null_type>
struct typelist
{
typedef H Head;
typedef T Tail;
};
template<typename T, typename TList> struct exists;
template<typename T, typename Tail>
struct exists<T, typelist<T, Tail> >
{
static const bool value = true;
};
template<typename T, typename Head, typename Tail>
struct exists<T, typelist<Head, Tail> >
{
static const bool value = false || exists<T, Tail>::value;
};
template<typename T>
struct exists<T, null_type >
{
static const bool value = false;
};
template<bool>
struct compile_time_error;
template<>
struct compile_time_error<true> {};
-- -
Now follows the testing code: 现在遵循测试代码:
//////////////////////////////////////////////////////////////
//usage
typedef typelist<int> t1;
typedef typelist<short, t1> t2;
typedef typelist<char, t2> t3;
typedef typelist<unsigned char, t3> t4;
typedef t4 supported_types;//supported_types: int, short, char, unsigned char
template<typename T>
struct X
{
compile_time_error<exists<T,supported_types>::value> unsupported_type_used;
};
int main() {
//testing if exists<> work or not!
cout <<(exists<int,supported_types>::value)<< endl; //should print 1
cout <<(exists<unsigned int,supported_types>::value)<<endl;//should print 0
cout <<(exists<char,supported_types>::value)<< endl; //should print 1
cout <<(exists<long,supported_types>::value)<< endl; //should print 0
X<int> x1; //okay - int is supported!
//X<long> x2; //error - long is unsupported!
return 0;
}
which compiles perfectly fine ( ideone ), and gives this output (for the cout
statements): 编译完全正常( ideone ),并给出此输出(对于
cout
语句):
1
0
1
0
But if you uncomment the line X<long> x2;
但如果你取消注释
X<long> x2;
in the above code, it will not compile, since long
is an unsupported type. 在上面的代码中,它不会编译,因为
long
是不受支持的类型。 And it gives this error, which is easy to read and understand ( ideone ): 它给出了这个错误,易于阅读和理解( ideone ):
prog.cpp: In instantiation of 'X':
prog.cpp:在'X'的实例化中:
prog.cpp:68: instantiated from hereprog.cpp:68:从这里实例化
prog.cpp:56: error: 'X::unsupported_type_used' has incomplete typeprog.cpp:56:错误:'X :: unsupported_type_used'的类型不完整
prog.cpp:38: error: declaration of 'struct compile_time_error'prog.cpp:38:错误:'struct compile_time_error'的声明
Hope this helps you. 希望这对你有所帮助。
Now you can write a class template called enable_if_supported
which takes two type arguments: T
and supported_types
. 现在您可以编写一个名为
enable_if_supported
的类模板,它带有两个类型参数: T
和supported_types
。 You can derive your class from enable_if_supported
as: 您可以从
enable_if_supported
派生您的类:
template<typename T>
struct X : enable_if_supported<T, supported_types>
{
//your code
};
This looks a bit clean. 这看起来有点干净。
enable_if_supported
class template now is defined in the framework section. 现在,
enable_if_supported
类模板在框架部分中定义。 See it here working : http://www.ideone.com/EuOgc 请在此处查看: http : //www.ideone.com/EuOgc
How about this : 这个怎么样 :
#include <string>
template <typename T>
struct good_type;
template <>
struct good_type< std::string > {};
template <>
struct good_type< int > {};
template < typename T >
struct X
{
good_type< T > someVar;
};
int main(int argc, char** argv)
{
X< std::string > x1;
X< int > x2;
X<float> x3;
}
produces : 产生:
main.cpp: In instantiation of ‘X<float>’:
main.cpp:22:12: instantiated from here
main.cpp:15:18: error: ‘X<T>::someVar’ has incomplete type
main.cpp:4:8: error: declaration of ‘struct good_type<float>’
One more way : 还有一种方法:
#include <string>
template <typename T>
struct good_type;
template <> struct good_type< std::string > {typedef char value;};
template <> struct good_type< int > {typedef char value;};
template < typename T >
struct X
{
typedef typename good_type< T >::value valid;
};
int main(int argc, char** argv)
{
X< std::string > x1;
X< int > x2;
X<float> x3;
}
will produce : 将产生:
main.cpp: In instantiation of ‘X<float>’:
main.cpp:19:12: instantiated from here
main.cpp:12:42: error: invalid use of incomplete type ‘struct good_type<float>’
main.cpp:4:8: error: declaration of ‘struct good_type<float>’
and you can create a macro to define valid types : 并且您可以创建一个宏来定义有效类型:
#define VALID( x ) template <> struct good_type< x > {typedef char value;}
VALID( std::string );
VALID( int );
BOOST_MPL_ASSERT
produces better messages: http://ideone.com/BR0GJ BOOST_MPL_ASSERT
产生更好的消息: http : BOOST_MPL_ASSERT
prog.cpp: In instantiation of ‘X<float>’:
prog.cpp:24: instantiated from here
prog.cpp:17: error: no matching function for call to
‘assertion_failed(mpl_::failed************ good_type<float>::************)’
prog.cpp: In function ‘int main(int, char**)’:
prog.cpp:22: warning: unused variable ‘x1’
prog.cpp:23: warning: unused variable ‘x2’
prog.cpp:24: warning: unused variable ‘x3’
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