限制模板功能 [英] Restrict Template Function

问题描述

我在 http://codepad.org/ko8vVCDF 上撰写了一个使用模板功能的示例程序。

I wrote a sample program at http://codepad.org/ko8vVCDF that uses a template function.

如何将模板函数限制为只使用数字？ （int，double等）

How do I retrict the template function to only use numbers? (int, double etc.)

#include <vector>
#include <iostream>

using namespace std;

    template <typename T>
T sum(vector<T>& a)
{
    T result = 0;
    int size = a.size();
    for(int i = 0; i < size; i++)
    {
        result += a[i];
    }

    return result;
}

int main()
{
    vector<int> int_values;
    int_values.push_back(2);
    int_values.push_back(3);
    cout << "Integer: " << sum(int_values) << endl;

    vector<double> double_values;
    double_values.push_back(1.5);
    double_values.push_back(2.1);
    cout << "Double: " << sum(double_values);

    return 0;
}

推荐答案

模板是为了使它使用你想要的类型的东西，其他类型没有。

The only way to restrict a template is to make it so that it uses something from the types that you want, that other types don't have.

所以，你构造一个int，使用+和+ =，调用一个复制构造函数等。

So, you construct with an int, use + and +=, call a copy constructor, etc.

任何类型的所有这些都将与你的函数一起工作 - 所以，如果我创建一个新类型这些特性，你的函数会工作 - 这是伟大的，不是吗？

Any type that has all of these will work with your function -- so, if I create a new type that has these features, your function will work on it -- which is great, isn't it?

如果你想限制它，更多的功能，只使用定义

If you want to restrict it more, use more functions that only are defined for the type you want.

另一种实现方法是创建一个traits模板 - 类似这样

Another way to implement this is by creating a traits template -- something like this

template<class T>
SumTraits
{
public:
  const static bool canUseSum = false;
}

然后专门为你想要的类确定：

And then specialize it for the classes you want to be ok:

template<>
class SumTraits<int>
{
  public:
    const static bool canUseSum = true;
};

然后在您的代码中，您可以写

Then in your code, you can write

if (!SumTraits<T>::canUseSum) {
   // throw something here
}

编辑：如注释中所述，您可以使用BOOST_STATIC_ASSERT使其成为编译时检查，而不是运行时检查

edit: as mentioned in the comments, you can use BOOST_STATIC_ASSERT to make it a compile-time check instead of a run-time one

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