81 lines
3.5 KiB
Markdown
81 lines
3.5 KiB
Markdown
CPP Primer Chp13 Exercises
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==========================
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> Exercise 13.1: What is a copy constuctor? When is it used?
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拷贝构造函数(copy constructor)是在用拷贝的方式初始化对象的时候，被调用的构造函数，它的形参必须是`const T&`类型，其中`T`为当前类的类名。它会在三种情形下被使用：
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+ 拷贝初始化(copy initialization)。比如说
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```cpp
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myClass object1();//default construction
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myClass object2 = object1;//copy initialization
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```
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+ 函数的形参是一个对象而非对象的引用。此时需要调用拷贝构造函数，从实参拷贝构造一个局部的形参传递到函数体的内部。
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+ 函数的返回值是一个对象而非对象的引用。由于函数内部的变量具有临时性，函数返回后就会被销毁，因此需要拷贝构造一个新的对象传递给函数的调用者。
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> Exercise 13.2: Explain why the following declaration is illegal:
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```cpp
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Sales_data::Sales_data(Sales_data rhs);
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```
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拷贝构造函数的参数必须是对对象的引用。这是因为拷贝构造函数的一个作用，就是在函数的参数为对象的非引用类型时，将实参拷贝构造到形参。如果如上面的声明，对于传入的`rhs`的实参，由于并非是引用类型，需要调用拷贝构造函数将其转化为形参，这个过程将一直持续下去，无法终止。
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> Exercise 13.4: Assuming Point is a class type with a public copy constructor, identify each use of the copy constructor in this program fragment:
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```cpp
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Point global;
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Point foo_bar(Point arg)
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{
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Point local = arg, *heap = new Point(global);
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*heap = local;
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Point pa[4] = { local, *heap };
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return *heap;
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}
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```
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0. `arg`形参并非引用类型，参数传递对应了第一次调用。
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1. `Point local = arg`。拷贝初始化(copy initialization)时调用拷贝构造函数。
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2. `heap = new Point(global)`。显式调用拷贝构造函数。
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3. `Point pa[4] = { local, *heap };`这里有两次调用。
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4. `return *heap;`返回值并非引用类型，需要调用拷贝构造函数。
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> Given the following sketch of a class, write a copy constructor that copies all the members. Your constructor should dynamically allocate a new string and copy the object to which ps points, rather than copying ps itself.
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```cpp
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class HasPtr{
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public:
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HasPtr(const std::string &s = std::string()):
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ps(new std::string(s)), i(0){}
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private:
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std::string *ps;
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int i;
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};
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```
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```cpp
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//copy constructor
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HasPtr::HasPtr(const HasPtr& ptr){
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ps = new string(*ptr.ps);
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i = ptr.i;
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}
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```
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> Exercise 13.6: What is a copy-assignment operator? When is this operator used? What does the synthesized copy-assignment operator do? When is it synthesized?
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拷贝赋值运算符是一个函数，用于将形参对象的值拷贝到当前对象中；当需要对对象进行赋值时，应该使用拷贝复制运算符；默认的拷贝赋值运算符将传入对象的所有非静态(nonstatic)成员拷贝目标对象当中，可以看出这是一个浅拷贝(shallow copy)，当成员变量有指针类型时就会出现问题；如果用户没有定义自己的拷贝赋值运算符，编译器就会生成默认的拷贝赋值运算符。
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> Exercise 13.8: Write the assignment operator for the HasPtr class from exercise 13.5 in. As with the copy constructor, your
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assignment operator should copy the object to which ps points.
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```cpp
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HasPtr& HasPtr::operator=(const HasPtr& ptr){
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delete ps;
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ps = new string(*ptr.ps)
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i = ptr.i;
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return *this;
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}
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```
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