c++ - 模板特化问题

Question

我想调用template<typename T> foo(T x)并手动处理这些情况:T = std::vector<U> , T = std::string , T =任何其他情况。

这是我为此写的:

#include <iostream>
#include <vector>
#include <string>

template<typename T> void foo_impl(const std::string &data, std::string *) {
  std::cout << "foo for std::string called\n";
}

template<typename T> void foo_impl(const T &data, T *) {
  std::cout << "foo for general types called\n";
}

template<typename T> void foo_impl(const std::vector<T> &data, std::vector<T> *) {
  std::cout << "foo for std::vector<T> called\n";
}

template<typename T> void foo(const T &data) {
  foo_impl(data, static_cast<T*>(nullptr));
}


int main() {
  int i = 1;
  foo(i);
  std::vector<int> a = {0, 1};
  foo(a);
  std::string s = "abcd";
  foo<std::string>(s);
  return 0;
}

然而，foo(std::string x)在“T 是任何其他类型”的情况下被调用。我该如何处理？

Answer 1

对于模板:

template<typename T> void foo(const T &data) {
  std::cout << "foo for general types called\n";
}

以下是特化:

template<> void foo<>(const std::string &data) {
    std::cout << "foo for std::string called\n";
}

但简单的重载似乎更合适:

void foo(const std::string &data) {
    std::cout << "foo for std::string called\n";
}

由于函数无法进行部分特化，因此您必须对 vector 情况进行重载:

template<typename T, typename Alloc> void foo(const std::vector<T, Alloc> &data) {
  std::cout << "foo for std::vector<T, Alloc> called\n";
}

另一种方法是转发到可以(部分)专门化的类/结构:

template <typename T>
struct foo_impl {
    void operator (const T&) const
    {
        std::cout << "foo for general types called\n";
    }
};

// specialization for std::string
template <>
struct foo_impl<std::string>
{
    void operator (const T&) const
    {
        std::cout << "foo for std::string called\n";
    }
};

// partial specialization for std::vector
template <typename T, typename A>
struct foo_impl<std::vector<T, A>>
{
    void operator (const std::vector<T, A>&) const
    {
        std::cout << "foo for std::vector<T, A> called\n";
    }
};

template <typename T>
void foo(const T& t)
{
    foo_impl<T>{}(t);
}