Use object to manage resources
- 获得资源后立刻放入管理对象
- 管理对象运用析构函数确保资源被释放
1
2
3
4
5
6
7
8
9
10
11
12
13
|
void f() {
Investment* pInv = createInvestment();
// other operations
// 可能因为一系列原因导致delete无法执行
delete pInv; // release pointer
}
// manage resource by auto_ptr
void f2() {
std::auto_ptr<Investment> pInv(createInvestment());
// auto_ptr will automatically deconstruct and delete pInv
}
|
由于auto_ptr销毁时会自动删除所指之物，所以一定不能让多个auto_ptr指向同一对象。
若通过复制构造函数或复制操作符复制auto_ptr，它们会变成 null，而复制所得的指针将取得资源的唯一拥有权
1
2
3
|
std::auto_ptr<Investment> pInv1(createInvestment());
std::auto_ptr<Investment> pInv2(pInv1); // pInv1 becomes null
pInv1 = pInv2; // pInv2 becomes null
|
auto_ptr的替代方案是"引用计数型智慧指针"
1
2
3
4
5
|
void f() {
std::tr1::shared_ptr<Investment> pInv1(createInvestment());
std::tr1::shared_ptr<Investment> pInv2(pInv1);
pInv1 = pInv2;
}
|
auto_ptr & tr1::shared_ptr两者都在其析构函数中做delete而不是delete[]操作。
Think carefully about copying behavior in resource-managing classes
- 禁止复制
- 对底层资源使用"引用计数法"
shared_ptr
- 复制底部资源
- 转移底部资源的拥有权
auto_ptr
Provide access to raw resources in resource-managing classes
1
2
3
|
int daysHeld(const Investment *pi);
int days = daysHeld(pInv.get()); // return raw resource
|
提供显式转换函数
1
2
3
4
5
6
7
8
9
10
11
12
13
|
FontHandle getFont();
void releaseFont(FontHandle fh);
class Font {
public:
explicit Font(FontHandle fh) : f(fh) {
}
~Font() {
releaseFont(f);
}
private:
FontHandle f;
}
|
提供隐式转换函数
1
2
3
4
5
6
7
8
|
class Font {
public:
operator FontHandle() const {
return f;
}
private:
FontHandle f;
}
|
1
2
3
4
5
|
std::string* stringPtr1 = new std::string;
std::string* stringPtr2 = new std::string[100];
delete stringPtr1;
delete [] stringPtr2;
|
Store new objects in smart pointer in standalone statements
1
2
3
4
5
6
7
8
9
10
|
int priority();
void processWidget(std::tr1::shared_ptr<Widget> pw, int priority);
processWidget(std::tr1::shared_ptr<Widget>(new Widget), priority());
// 1. 执行 "new Widget"
// 2. 调用 priority => 如果抛出异常，new Widget返回的指针将会遗失
// 3. 调用 tr1::shared_ptr 构造函数
std::tr1::shared_ptr<Widget> pw(new Widget);
processWidget(pw, priority());
|
