[英]const reference to a function in a template parameter?
How do I declare that I want a const function reference (in a template parameter)? 如何声明要使用const函数引用(在模板参数中)? eg,
例如,
template< bool (&func)(int arg) >
void foo(int stuff);
but const? 但是const?
More concretely, if I try to compile the following with icpc
: 更具体地说,如果我尝试使用
icpc
编译以下内容:
template<bool (&func)(int arg)>
bool id(int arg) {
return func(arg);
}
class Foo {
public:
Foo() {};
virtual ~Foo() {};
bool bar(int arg) { return true; }
bool bar2(int arg) {
return id<bar>(arg);
};
};
int main() {
return 0;
}
I get 我懂了
$ icpc foo.cpp
foo.cpp(12): error: no instance of function template "id" matches the argument list
argument types are: (int)
return id<bar>(arg);
^
compilation aborted for foo.cpp (code 2)
or, with g++
, I get 或者,使用
g++
,我得到
$ g++ foo.cpp
foo.cpp: In member function ‘bool Foo::bar2(int)’:
foo.cpp:13:23: error: no matching function for call to ‘id(int&)’
return id<bar>(arg);
^
foo.cpp:13:23: note: candidate is:
foo.cpp:2:6: note: template<bool (& func)(int)> bool id(int)
bool id(int arg) {
^
foo.cpp:2:6: note: template argument deduction/substitution failed:
foo.cpp:13:23: error: could not convert template argument ‘Foo::bar’ to ‘bool (&)(int)’
return id<bar>(arg);
^
But if instead I move bar to toplevel, as in 但是,如果我改为将栏移到顶层,例如
template<bool (&func)(int arg)>
bool id(int arg) {
return func(arg);
}
bool bar(int arg) { return true; }
class Foo {
public:
Foo() {};
virtual ~Foo() {};
bool bar2(int arg) {
return id<bar>(arg);
};
};
int main() {
return 0;
}
it compiles fine. 它编译良好。 Why does this happen, and how can I fix it without making bar a global?
为什么会发生这种情况?如何在不将bar设置为全局的情况下解决该问题?
Note: In my original code, I was getting "(not const-qualified) cannot be initialized with a value of type" errors: (with icpc
) 注意:在我的原始代码中,我得到“(不能用类型值初始化)(错误)”错误:(使用
icpc
)
CollisionWorld.cpp(73): error: a reference of type "bool (&)(const Line &, vec_dimension={double}, vec_dimension={double}, vec_dimension={double}, vec_dimension={double})" (not const-qualified) cannot be initialized with a value of type "bool (const Line &, vec_dimension={double}, vec_dimension={double}, vec_dimension={double}, vec_dimension={double})"
QuadTree<Line, vec_dimension, line_inside_box_with_time> *quad_tree =
^
(with g++
) (使用
g++
)
CollisionWorld.cpp:73:58: error: could not convert template argument ‘CollisionWorld::line_inside_box_with_time’ to ‘bool (&)(const Line&, double, double, double, double)’
QuadTree<Line, vec_dimension, line_inside_box_with_time> *quad_tree =
^
The problem is that the template expects a free function, not a member funciton. 问题在于模板需要自由功能,而不是成员功能。 That is why it works when you put bar() out of Foo,
这就是为什么当您将bar()放到Foo中时它起作用的原因,
Try it like this: 像这样尝试:
template<typename C, bool (C::*func)(int arg)>
bool id(C *mthis, int arg) {
return (mthis->*func)(arg);
}
class Foo {
public:
Foo() {};
virtual ~Foo() {};
bool bar(int arg) { return true; }
bool bar2(int arg) {
return id<Foo, &Foo::bar>(this, arg);
};
};
To call a member function, you need two things: the this
pointer and the function. 要调用成员函数,您需要做两件事:
this
指针和函数。 Therefore it is not possible as easy as you write it. 因此,不可能像编写它那样容易。
id
would need the this
pointer! id
将需要this
指针!
The template definition would look like this: 模板定义如下所示:
template<bool (Foo::*func)(int)>
But still, you cannot implement a true id
function that works both on functions and member functions. 但是,您仍然不能实现同时在函数和成员函数上起作用的true
id
函数。
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