关于模板：基于成员存在的c ++条件编译

c++ conditional compilation based on member presence

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Possible Duplicate:
Is it possible to write a C++ template to check for a function's existence?

在JavaScript等语言中，您可以检查是否存在属性

1 2	// javascript if( object['property'] ) // do something

在C++中，我想根据EDCOX1的类型0是否具有某个属性来编译编译条件。这有可能吗？

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template <typename T>
class IntFoo
{
T container ;
public:
void add( int val )
{
// This doesn't work, but it shows what I'm trying to do.
// if the container has a .push_front method/member, use it,
// otherwise, use a .push_back method.
#ifdef container.push_front
container.push_front( val ) ;
#else
container.push_back( val ) ;
#endif
}

void print()
{
for( typename T::iterator iter = container.begin() ; iter != container.end() ; ++iter )
printf("%d", *iter ) ;

puts("
--end" ) ;
}
} ;

int main()
{
// what ends up happening is
// these 2 have the same result (500, 200 --end).
IntFoo< vector<int> > intfoo;
intfoo.add( 500 ) ;
intfoo.add( 200 ) ;
intfoo.print() ;

// expected that the LIST has (200, 500 --end)
IntFoo< list<int> > listfoo ;
listfoo.add( 500 ) ;
listfoo.add( 200 ) ; // it always calls .push_back

listfoo.print();
}

相关讨论

下面的示例演示了一种可用于(在C++ 11中使用表达式的SIMEAE)的技术：

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typedef char no;
typedef no (&yes)[2];

no detect_push_front( ... );

template < class C >
auto detect_push_front( C &c ) -> typename std::conditional< false, decltype( c.push_front( std::declval< typename C::value_type >() ) ), yes >::type;

template < class C >
struct has_push_front : std::integral_constant< bool, sizeof( detect_push_front( std::declval< C & >() ) ) == sizeof( yes ) > {};

template < class C >
void add( C &c, typename C::value_type v, std::true_type )
{
c.push_front( v );
}

template < class C >
void add( C &c, typename C::value_type v, std::false_type )
{
c.push_back( v );
}

template < class C >
void add( C &c, typename C::value_type v )
{
add( c, v, has_push_front< C >() );
}

int main()
{
std::vector< int > v;
add( v, 0 );
add( v, 1 );
add( v, 2 );
std::copy( v.begin(), v.end(), std::ostream_iterator< int >( std::cout,"" ) );
std::cout << '
';

std::list< int > l;
add( l, 0 );
add( l, 1 );
add( l, 2 );
std::copy( l.begin(), l.end(), std::ostream_iterator< int >( std::cout,"" ) );
std::cout << '
';
}

输出：

1 2	0 1 2 2 1 0