在C ++中如何使用模板函数作为std :: for_each中的第3个参数？

I am trying to use std::for_each to output the contents of vectors, which may contain different types. So I wrote a generic output function like so:

我正在尝试使用std :: for_each来输出向量的内容,这些向量可能包含不同的类型。所以我写了一个通用输出函数,如下所示:

template<typename T> void output(const T& val)
{
    cout << val << endl;
}

which I would like to use with:

我想用它:

std::for_each(vec_out.begin(), vec_out.end(), output);

but the compiler complains with "could not deduce template argument" in the for_each statement. Also complains with "A function template cannot be an argument to another function template".

但是编译器在for_each语句中抱怨“无法推断出模板参数”。还抱怨“函数模板不能成为另一个函数模板的参数”。

Is this not possible? I would have thought the compiler would know the type of vec_out (it's vector) and so should instantiate the function "output(const double& val)"?

这不可能吗?我原以为编译器会知道vec_out的类型(它的向量),所以应该实例化函数“output(const double&val)”?

If this doesn't work how can I get similar STL functionality without writing manual loops?

如果这不起作用,如何在不编写手动循环的情况下获得类似的STL功能?

I am quite new to C++ and still learning the ropes :-)

我对C ++很新,还在学习绳索:-)

3 个解决方案

#1

Try:

std::for_each(vec_out.begin(), vec_out.end(), output<T>);

where vec_out is a container (vector) of type T.

其中vec_out是T类型的容器(向量)。

Note: The for_each algorithm expects an unary functor for its last argument. See the link for an example using functors.

注意:for_each算法需要一个一元仿函数用于其最后一个参数。有关使用仿函数的示例,请参阅链接。

#2

You have to pass an instantiation of the template. Something like output<int> if your vector is vector of integers.

您必须传递模板的实例化。如果你的向量是整数向量,那就像输出。

For example:

template<typename T> void output(const T& val)
{
    cout << val << endl;
}



void main(int argc,char *argv[])
{
    std::vector<int> vec_out;
    std::for_each(vec_out.begin(), vec_out.end(), output<int>);
}

#3

I'd just like to add to the correct answers: the compiler can deduce the type if you wrap up your template function in a function object (aka functor):

我只想添加正确的答案:如果你将模板函数包装在一个函数对象(aka functor)中,编译器可以推断出类型:

struct OutputFunctor
{
  template <typename T>
  void operator()(const T& val) const { output(val); }
};

void test()
{
  std::vector<int> ints;
  std::vector<float> floats;

  std::for_each(ints.begin(), ints.end(), OutputFunctor());
  std::for_each(floats.begin(), floats.end(), OutputFunctor());
}

#1