如何解决局部专业化的默认参数
| 我想在用户端实现什么语法:double a(1.),b(2.),deps(.1);
bool res1 = compare<double>()(a,b); // works with default eps
bool res2 = compare<double,&deps>()(a,b); // works with explicitly provided eps
float c(1.),d(1.). feps(.1);
bool res3 = compare<float>()(c,d); // don\'t have default eps - must not compile
bool res4 = compare<float,&feps>()(c,d); // works only with provided eps
我现在有什么实现方式(由于不允许部分专业化的默认参数,因此无法正常工作):
extern double eps_double; // somewhere defined and initialized
template<typename T,const T* eps>
struct compare { // actually inherits std::binary_function
bool operator()(const T& t1,const T& t2) {
return t1 < t2 - *eps;
}
};
template<const double* eps = &eps_double>
struct compare<double,eps> { // the same as in default implementation
};
我尝试使用具有静态成员的enable_if和wrapper类,但是不能将静态成员分配给extern变量;
更新:
实际的问题是通用结构和专用结构的名称相等。我不知道如何重命名它:
// treats all explicitly passed eps and don\'t need default parameter
template<typename T,const T* eps>
struct compare_eps { // need another name!
bool operator()(const T& t1,const T& t2) {
return t1 < t2 - *eps;
}
};
// don\'t need default parameter either
// because we always kNow what eps to use for a concrete type
template<typename T>
struct compare {
// define nothing -> will not compile on types we don\'t have specialization for
};
template<>
struct compare<double> {
// use here eps_double hardcoded
};
解决方法
我不知道你为什么认为这有意义
compare<double,deps>
您无法做到这一点:模板参数不能是double
类型的值(它们可以是double
类型的左值,但是您的模板需要double
的地址,因此\处于关闭状态)。
您可以使用功能模板来使语法正常工作
extern double eps_double;
template<typename T>
types::compare<T,&eps_double> compare(
typename enable_if<is_same<T,double>>::type * = 0
) {
return types::compare<T,&eps_double>();
}
template<typename T,const T *eps>
types::compare<T,eps> compare() {
return types::compare<T,eps>();
}
另外,如果您打算接受一些丑陋的技巧,则可以使用类模板
template<typename T,const T* eps = &eps_double>
struct compare {
bool operator()(const T& t1,const T& t2) {
return t1 < t2 - *eps;
}
};
如果您同时提供两个参数,则将不使用默认参数。如果仅提供“ 9”,则将使用默认参数并将其起作用。如果仅提供<float>
,则也将使用默认参数,但将不起作用。
,您需要更改拥有比较运算符的结构,以便可以专门化外部模板,请参见:http://ideone.com/xqtjz
代码是:
extern double eps_double; // somewhere defined and initialized
extern double deps; // NOTE: you have to make these extern a well,else cannot work
extern float feps;
template<typename T>
struct compare {
// this internal structure now has the operator()
template <const T* eps>
struct it
{
bool operator()(const T& t1,const T& t2) const {
return t1 < t2 - *eps;
}
};
};
// specialize compare for double
template<>
struct compare<double>
{
// here you can provide the default
template<const double* eps=&eps_double>
struct it
{
bool operator()(const double& t1,const double& t2)
{
return t1 < t2 - *eps;
}
};
};
int main(void)
{
double a(1.),b(2.);
bool res1 = compare<double>::it<>()(a,b); // works with default eps
bool res2 = compare<double>::it<&deps>()(a,b); // works with explicitly provided eps
float c(1.),d(1.);
bool res3 = compare<float>::it<>()(c,d); // don\'t have default eps - will not compile
bool res4 = compare<float>::it<&feps>()(c,d); // works only with provided eps
}
,我会采用类似特征的方法:
template<class T>
struct DefaultEps;
template<>
struct DefaultEps<double>
{
static const double eps = 4.0;
};
// may or may not be needed in .cpp
// const double DefaultEps<double>::eps;
template<>
struct DefaultEps<float>
{
static const float eps = 4.0;
};
// may or may not be needed in .cpp
// const float DefaultEps<float>::eps;
template<class T,class E = DefaultEps<T> >
struct Compare
{
bool operator()(T const &t1,T const &t2)
{
return(t1 < t2 - E::eps);
}
};
然后在需要特定的epsilon时:
struct SpecificEps
{
static const float eps = 4.0;
};
// may or may not be needed in .cpp
// const float SpecificEps::eps;
并使用它:
Compare<float,SpecificEps>()(...);
版权声明:本文内容由互联网用户自发贡献,该文观点与技术仅代表作者本人。本站仅提供信息存储空间服务,不拥有所有权,不承担相关法律责任。如发现本站有涉嫌侵权/违法违规的内容, 请发送邮件至 dio@foxmail.com 举报,一经查实,本站将立刻删除。