局部专业化的默认参数
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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
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
};
// 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
};
没有找到相关结果
已邀请:
3 个回复
授巨
您无法做到这一点：模板参数不能是类型的值（它们可以是类型的左值，但是您的模板需要的地址，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>(); }template<typename T, const T* eps = &eps_double> struct compare { bool operator()(const T& t1, const T& t2) { return t1 < t2 - *eps; } };，则也将使用默认参数，但将不起作用。土投
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 }暑袜眠退
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); } };struct SpecificEps { static const float eps = 4.0; }; // may or may not be needed in .cpp // const float SpecificEps::eps;