问题描述
这是 C ++ Primer 5th Edition 的练习:
template <typename T> class Stack { };
void f1(Stack<char>); // (a)
class Exercise {
Stack<double> &rsd; // (b)
Stack<int> si; // (c)
};
int main() {
Stack<char> *sc; // (d)
f1(*sc); // (e)
int iObj = sizeof(Stack< string >); // (f)
}
以下是我尝试过的内容:
Below is what I tried:
(a)Stack<char>被实例化，但没有成员被实例化.
(a) Stack<char> is instantiated , but no member of it is instantiated.
(b)Stack<double>已实例化，但没有成员实例化.
(b) Stack<double> is instantiated , but no member of it is instantiated.
(c)Stack<int>及其默认构造函数被实例化.
(c) Stack<int> and its default constructor are instantiated.
(d)(e)完全不知道...
(d) (e) totally no idea...
(f)Stack< string >已实例化，但没有成员实例化.
(f) Stack< string > is instantiated , but no member of it is instantiated.
我是对的吗?谁能告诉我该代码是如何实例化的?
Am I right? Can anyone tell me how this code is instantiated?
推荐答案
在您的特定情况下，声明并不意味着实例化
In your specific case a declaration doesn't mean an instantiation
#include <iostream>
using namespace std;
template <typename T> class Stack {
typedef typename T::ThisDoesntExist StaticAssert; // T::NotExisting doesn't exist at all!
};
void f1(Stack<char>); // No instantiation, compiles
class Exercise {
Stack<double> &rsd; // No instantiation, compiles (references don't need instantiation, are similar to pointers in this)
Stack<int> si; // Instantiation! Doesn't compile!!
};
int main(){
Stack<char> *sc; // No Instantiation, this compiles successfully since a pointer doesn't need instantiation
f1(*sc); // Instantiation of Stack<char>! Doesn't compile!!
int iObj = sizeof(Stack< std::string >); // Instantiation of Stack<std::string>, doesn't compile!!
}
注意指针/引用的东西:它们不需要实例化，因为实际上没有分配数据(指针只是几个字节来包含地址，不需要存储所有数据.) pimpl习惯用语).
notice the pointer/reference stuff: they don't require instantiation since no data is actually allocated (a pointer is just a few bytes to contain the address, has no need to have all the data stored.. take a look at the pimpl idiom ).
仅当分配了东西时，才必须完全解析模板(而且这种情况发生在编译时，这就是为什么它们通常都需要声明和定义的原因.尚无链接阶段)
Only when stuff is allocated then the template has to be completely resolved (and that happens at compile-time, that's why they usually need both declaration and definition.. there's no linking phase yet)
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