成员函数实例化 [英] Member function instantiation
问题描述
以下在 GCC 4.8.1 上编译(使用 --std=c++11
):
struct non_default_constructible { non_default_constructible() = delete;};模板结构虚拟{T new_t() { 返回 T();}};int main(int argc, char** argv) {虚拟d;返回0;}
棘手的部分是 dummy
显然格式错误,但这并不能阻止编译器实例化 dummy
.
这是标准规定的行为吗?什么是相关的部分/关键字?
类模板的成员函数仅在上下文需要时才会实例化,这意味着您在尝试使用 new_t 之前不会看到任何错误()
.C++ 标准的相关部分是:
§ 14.7.1 隐式实例化 [temp.inst]
除非函数模板特化已被显式实例化或显式特化,函数模板特化在需要函数定义存在的上下文中被引用时隐式实例化.除非调用是对函数模板显式特化或显式特化类模板的成员函数,否则函数模板的默认参数或类模板的成员函数在需要默认参数的值.
[ 示例:
template
结构 Z {无效 f();无效 g();};无效 h() {Z 一种;//类 Z 的实例化必需的Z 字符 * p;//类 Z 的实例化不需要Z <double >* q;//类 Z 的实例化不需要a.f();//需要实例化 Z ::f()p->g();//类 Z 的实例化需要,和//需要实例化 Z ::g()} 此示例中没有任何内容需要
class Z
、Z
或::g() Z
是隐式的实例化.— 结束示例 ]::f()
The following compiles on GCC 4.8.1 (with --std=c++11
):
struct non_default_constructible { non_default_constructible() = delete; };
template<class T>
struct dummy {
T new_t() { return T(); }
};
int main(int argc, char** argv) {
dummy<non_default_constructible> d;
return 0;
}
The tricky part is that dummy<non_default_constructible>::new_t()
is obviously ill-formed, but that does not prevent the compiler from instantiating dummy<non_default_constructible>
.
Is this the behaviour specified by the standard? And what would be the relevant sections/keywords?
The member functions of a class template are instantiated only when required by a context, which means you will not see any error until you try to use new_t()
. The related section from the C++ standard is:
§ 14.7.1 Implicit instantiation
[temp.inst]
Unless a function template specialization has been explicitly instantiated or explicitly specialized, the function template specialization is implicitly instantiated when the specialization is referenced in a context that requires a function definition to exist. Unless a call is to a function template explicit specialization or to a member function of an explicitly specialized class template, a default argument for a function template or a member function of a class template is implicitly instantiated when the function is called in a context that requires the value of the default argument.
[ Example:
template<class T> struct Z { void f(); void g(); }; void h() { Z<int> a; // instantiation of class Z<int> required Z<char>* p; // instantiation of class Z<char> not required Z<double>* q; // instantiation of class Z<double> not required a.f(); // instantiation of Z<int>::f() required p->g(); // instantiation of class Z<char> required, and // instantiation of Z<char>::g() required }
Nothing in this example requires
class Z<double>
,Z<int>::g()
, orZ<char>::f()
to be implicitly instantiated. — end example ]
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