C ++模板陷阱 [英] C++ template gotchas

问题描述

刚刚我不得不通过网站找出为什么模板类模板成员函数给出语法错误：

just now I had to dig through the website to find out why template class template member function was giving syntax errors:

template<class C> class F00 {
   template<typename T> bar();
};
...
Foo<C> f;
f.bar<T>(); // syntax error here

现在我意识到模板括号被当作关系运算符。为了做到目的，需要以下奇怪的语法，cf http://stackoverflow.com/questions/1682844/templates-template-function-not-playing-well-with-classs-template-member-functi ：

f.template bar<T>();

你遇到的C ++ / C ++模板的其他奇怪的方面和秘诀考虑是常识？

what other bizarre aspects and gotcha of C++/C++ templates you have encountered that were not something that you would consider to be common knowledge?

推荐答案

我第一次从另一个模板类继承一个模板类， p>

I got tripped up the first time I inherited a templated class from another templated class:

template<typename T>
class Base {
    int a;
};

template<typename T>
class Derived : public Base<T> {
    void func() {
        a++; // error! 'a' has not been declared
    }
};

问题是编译器不知道 Base< T& 将成为默认模板或专用模板。专用版本可能不具有 int a 作为成员，因此编译器不会假定它可用。但你可以告诉编译器它将在那里与使用指令：

The problem is that the compiler doesn't know if Base<T> is going to be the default template or a specialized one. A specialized version may not have int a as a member, so the compiler doesn't assume that it's available. But you can tell the compiler that it's going to be there with the using directive:

template<typename T>
class Derived : public Base<T> {
    using Base<T>::a;
    void func() {
        a++; // OK!
    }
};

或者，您可以明确表示您正在使用 T ：

Alternatively, you can make it explicit that you are using a member of T:

void func {
    T::a++; // OK!
}

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