In C++11, I'd like to have a member variable in a class and a constructor for its initialization only if its default template value was chosen (only for supported types like int, of course).
有什么推荐的方法可以做到这一点(允许增压)？
就像是:
template< int _x = -1 > struct C {
C() {} // only available if _x != -1
C( int x ) : x( x ) {} // only available if _x == -1
// more methods that are common for all _x and refer to _x / x
private:
int x; // only available if _x == -1
// more members that are common for all _x
};
或者，换句话说:为了优化大小和速度，如果选择了模板默认值以外的其他值，则我想使用编译时间常数而不是存储在成员变量中的值。
--
这是使所有内容更清晰的示例:
template< int _size = -1 > struct Block {
Block() { buf = mmap( _size, ... ); } // exists only when size!=-1
Block( int s ) { buf = mmap( size = s, ... ); } // exists only when size==-1
~Block() { munmap( buf, getSize() ); } // should use the correct size
int getSize() const { return ???; } // gets _size if !=-1, size otherwise
// other methods that use buf and getSize()
private:
void *buf;
const int size; // only exists for size == -1!
};
这部分解决了它:
template< int _x > struct X {
int getX() const { return _x; }
};
template<> struct X< -1 > {
X( x ) : x( x ) {}
int getX() const { return _x; }
private:
int x;
};
template< int _x = -1 > struct C : X< _x > {
C() {} // only available if _x != -1
C( int x ) : X< _x >( x ) {} // only available if _x == -1
// more methods that are common for all _x and use this->getX()
};
但是
C的构造函数又如何呢？还有其他/更好的解决方案可用吗？ 最佳答案
只是一个想法，但也许会有所帮助:您可以尝试仅将基类用于最小的差异，并在不存在成员变量的情况下“伪造”成员变量以允许其余变量进行编译:
template< int _x > class B
{
public:
B() {}
protected:
static const int x = _x;
};
template<> class B< -1 >
{
public:
B( int i ) : x( i ) {}
protected:
int x;
};
template< int _x = -1 >
class C : public B<_x>
{
public:
using B<_x>::B; // inherit B's ctors
void f()
{
if ( x == ... ) // uses either the member variable x or the static const int x!
}
};
但正如我所说，这只是一个主意...