C ++:如何传递函数指针,存储在局部变量中,作为模板参数 [英] C++ : How to pass function pointer, stored in a local variable, as a template parameter
问题描述
using namespace std;
float test1(float i){
return i * i;
}
int test2(int i){
return i + 9;
}
struct Wrapper {
typedef void(* wrapper_type)(int);
template< class R,class A>
void wrap(string name,R(* fn)(A)){
wrapper_type f_ =& Wrapper :: wrapper1< R,A,fn&
//在真正的程序中,f_会在一个数组中传递给某个c库
wrappers [name] = f_;
}
void run(int i){
map< string,wrapper_type> :: iterator it,end = wrappers.end
for(it = wrappers.begin(); it!= end; it ++){
wrapper_type wt =(* it).second;
(* wt)(i);
}
}
template< class R,class A,R(* fn)(A)>
static void wrapper1(int mul){
// do something
A arg = mul;
R result =(* fn)(arg);
cout<< Result:<<结果< endl
}
map< string,wrapper_type>包装纸
};
int main(int argc,char ** argv){
Wrapper w;
w.wrap(test1,& test1);
w.wrap(test2,& test2);
w.run(89);
return 0;
}
这里是g ++错误:
main.cpp:31:error:'fn'不是类型'float(*)(float)'的有效模板参数,因为函数'fn'
从我的理解,问题是一个局部变量没有链接;因此它不能用作模板参数。
所以,我想知道是否有办法解决这个问题或另一种技术来完成同样的工作?
感谢。
编辑1:
我完全理解,我不能传递一个值,不能在编译时确定为一个模板参数。我要问的是 - 有更好的方法吗?现在对我有用的解决方案是:
template< class R,class A,R(* fn) >
void wrap(string name){
wrapper_type f_ =& Wrapper :: wrapper1< R,A,fn> ;;
//在实际程序中,f_将被传递到一个数组sm c库
wrappers [name] = f_;
}
并调用:
w.wrap< float,float,& test1>(test1);
w.wrap< int,int,& test2>(test2);
但这里我必须在包装期间传递参数类型。有什么地方可以避免这种情况吗?
编辑2:
只是为了澄清或添加更多信息:必须类似于LuaBind或Boost.Python ie Wrapper.wrap(test1,& test1);应该足够了。
模板参数必须在编译时已知,因此您必须重新设计您的代码, p>
编辑:为您更新的问题使用Boost function_traits。
template< R (* fn)(A)
void wrap(string name){
wrapper_type f_ =& Wrapper :: wrapper1< function_traits< A> :: result_type,function_traits< A> :: arg1_type,fn> ;;
//在实际程序中,f_将被传递到一个数组sm c库
wrappers [name] = f_;
}
using namespace std;
float test1(float i){
return i * i;
}
int test2(int i){
return i+9;
}
struct Wrapper{
typedef void (*wrapper_type)(int);
template<class R, class A>
void wrap(string name,R (*fn) (A) ){
wrapper_type f_ = &Wrapper::wrapper1<R,A,fn>;
// in the real program, f_ would be passed in an array to some c library
wrappers[name] = f_;
}
void run(int i){
map<string,wrapper_type>::iterator it, end = wrappers.end();
for(it=wrappers.begin();it!=end;it++){
wrapper_type wt = (*it).second;
(*wt)(i);
}
}
template<class R, class A, R (*fn) (A)>
static void wrapper1(int mul){
//do something
A arg = mul;
R result = (*fn)( arg );
cout << "Result: " << result << endl;
}
map<string,wrapper_type> wrappers;
};
int main(int argc, char** argv) {
Wrapper w;
w.wrap("test1",&test1);
w.wrap("test2",&test2);
w.run(89);
return 0;
}
Here's the g++ error:
main.cpp:31: error: ‘fn’ is not a valid template argument for type ‘float (*)(float)’ because function ‘fn’ has not external linkage
From what i understand, the problem is that a local variable has no linkage; thus it can not be used as a template parameter.
So, i wanted to know if there's a way to get around this problem or another technique to accomplish the same ?
Thanks.
Edit 1:
I totally understand that I can't pass a value that can not be determined at compile time as a template paramter. What i'm asking is - is there a better way to do this? Right now the solution that works for me is :
template<class R, class A,R (*fn) (A)>
void wrap(string name){
wrapper_type f_ = &Wrapper::wrapper1<R,A,fn>;
// in the real program, f_ would be passed in an array to sm c library
wrappers[name] = f_;
}
and called as :
w.wrap<float, float, &test1>("test1");
w.wrap<int, int, &test2>("test2");
But here I've to pass argument-type during wrapping. Is there someway to avoid this ?
EDIT 2:
Just to clarify or add more info: The interface I want to present to the user has to be similar to LuaBind or Boost.Python i.e. Wrapper.wrap("test1",&test1); should be sufficient.
Template parameters must be known at compile time, so you'll have to redesign your code taking this into account.
Edit: for your updated question use Boost function_traits.
template<R (*fn) (A)>
void wrap(string name){
wrapper_type f_ = &Wrapper::wrapper1<function_traits<A>::result_type, function_traits<A>::arg1_type,fn>;
// in the real program, f_ would be passed in an array to sm c library
wrappers[name] = f_;
}
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