T.69: Inside a template, don't make an unqualified non-member function call unless you intend it to be a customization point
T.69:在模板内部,不要进行不受限制的非成员函数调用,除非你希望它成为一个定制点
Example(示例)
There are three major ways to let calling code customize a template.
存在三种主要的方式让调用代码定制模板。
template<class T>
// Call a member function
void test1(T t)
{
t.f(); // require T to provide f()
}
template<class T>
void test2(T t)
// Call a non-member function without qualification
{
f(t); // require f(/*T*/) be available in caller's scope or in T's namespace
}
template<class T>
void test3(T t)
// Invoke a "trait"
{
test_traits<T>::f(t); // require customizing test_traits<>
// to get non-default functions/types
}
A trait is usually a type alias to compute a type, a constexpr function to compute a value, or a traditional traits template to be specialized on the user's type.
特征通常是一种用于计算类型的类型别名,一种用于求值的常量表达式函数,或者用于针对某个用户类型特化的传统的特征模板。
Note(注意)
If you intend to call your own helper function helper(t) with a value t that depends on a template type parameter, put it in a ::detail namespace and qualify the call as detail::helper(t);. An unqualified call becomes a customization point where any function helper in the namespace of t's type can be invoked; this can cause problems like unintentionally invoking unconstrained function templates.
如果你想用依赖模板类型参数的值t调用你自己的帮助函数helper(t),将它放入::detail命名空间并用detail::helper(t)对调用进行限定;如果一个帮助函数处于t的类型可以被触发的命名空间,不受限的调用会成为一个定制点;这会引起意外调用非约束函数模板等问题。
Enforcement(实施建议)