第六章:Initialization & Cleanup
时间：2010-05-04 来源：liuyun213
第六章:Initialization & Cleanup
Guaranteed initialization with the constructor
通过constructor保证initialization Guaranteed cleanup with the destructor
通过destructor保证cleanup
对象跑出作用域({})时会自动析构,即便是goto出作用域也会析构,但 nonlocal goto(如setjmp() and longjmp())时析构函数不会被调用 Elimination of the definition block
在C++里,定义不总是需要在头部,变量应该在需要时才进行定义. for loops
for后括号内定义的变量的作用域是紧随for的{}内 Storage allocation
空间分配:虽然c++的变量可以不再头部定义,在需要时再定义,但一般情况下 c++采取了和C一样的空间分配策略,在一开始就分配了全部空间.在作用域内 {},变量的定义与否是确定的,变量总是能被定义或不被定义,否则c++编译器或 发出警告或报告错误
如:
if(i < 10) {
//! goto jump1; // Error: goto bypasses init
}
X x1; // Constructor called here
jump1:
... ... Aggregate initialization
数组时同一类型的,class和struct是不同类型的聚合.
数组可以如下初始化:
int a[5] = { 1, 2, 3, 4, 5 }; // 逐个初始化
int b[6] = {0}; // 第一个正常初始化,其余初始化为0
int c[] = { 1, 2, 3, 4 };
数组元素个数计算: sizeof(array) / sizeof(*array)
C中的结构体元素是public的,一般可以如下初始化
struct X {
int i;
float f;
char c;
};
X x1 = { 1, 2.2, 'c' };
c++里面,struct和class基本一样,它们的初始化应该采用构造函数,如下:
struct Y {
float f;
int i;
Y(int a);
};
Y y1[] = { Y(1), Y(2), Y(3) }; Default constructors:默认构造函数
struct和class要使用合适的构造函数进行初始化
Y y2[2] = { Y(1) }; // 报错,找不到合适的构造函数
Y y3[7]; // 报错,同上
Y y4; // 报错,同上
构造函数的地位相当重要,当且仅当你不显示提供构造函数时,c++编译器会偷 偷地给你合成一个.但是它往往并不能按照你的意愿工作,所以你应该总是自己 提供一个构造函数,除非c++编译器默认生成的能够很好的工作. Summary
The seemingly elaborate mechanisms provided by C++ should give you a strong hint about the critical importance placed on initialization and cleanup in the language. As Stroustrup was designing C++, one of the first observations he made about productivity in C was that a significant portion of programming problems are caused by improper initialization of variables. These kinds of bugs are hard to find, and similar issues apply to improper cleanup. Because constructors and destructors allow you to guarantee proper initialization and cleanup (the compiler will not allow an object to be created and destroyed without the proper constructor and destructor calls), you get complete control and safety. Aggregate initialization is included in a similar vein – it prevents you from making typical initialization mistakes with aggregates of built-in types and makes your code more succinct. Safety during coding is a big issue in C++. Initialization and cleanup are an important part of this, but you’ll also see other safety issues as the book progresses.
通过constructor保证initialization Guaranteed cleanup with the destructor
通过destructor保证cleanup
对象跑出作用域({})时会自动析构,即便是goto出作用域也会析构,但 nonlocal goto(如setjmp() and longjmp())时析构函数不会被调用 Elimination of the definition block
在C++里,定义不总是需要在头部,变量应该在需要时才进行定义. for loops
for后括号内定义的变量的作用域是紧随for的{}内 Storage allocation
空间分配:虽然c++的变量可以不再头部定义,在需要时再定义,但一般情况下 c++采取了和C一样的空间分配策略,在一开始就分配了全部空间.在作用域内 {},变量的定义与否是确定的,变量总是能被定义或不被定义,否则c++编译器或 发出警告或报告错误
如:
if(i < 10) {
//! goto jump1; // Error: goto bypasses init
}
X x1; // Constructor called here
jump1:
... ... Aggregate initialization
数组时同一类型的,class和struct是不同类型的聚合.
数组可以如下初始化:
int a[5] = { 1, 2, 3, 4, 5 }; // 逐个初始化
int b[6] = {0}; // 第一个正常初始化,其余初始化为0
int c[] = { 1, 2, 3, 4 };
数组元素个数计算: sizeof(array) / sizeof(*array)
C中的结构体元素是public的,一般可以如下初始化
struct X {
int i;
float f;
char c;
};
X x1 = { 1, 2.2, 'c' };
c++里面,struct和class基本一样,它们的初始化应该采用构造函数,如下:
struct Y {
float f;
int i;
Y(int a);
};
Y y1[] = { Y(1), Y(2), Y(3) }; Default constructors:默认构造函数
struct和class要使用合适的构造函数进行初始化
Y y2[2] = { Y(1) }; // 报错,找不到合适的构造函数
Y y3[7]; // 报错,同上
Y y4; // 报错,同上
构造函数的地位相当重要,当且仅当你不显示提供构造函数时,c++编译器会偷 偷地给你合成一个.但是它往往并不能按照你的意愿工作,所以你应该总是自己 提供一个构造函数,除非c++编译器默认生成的能够很好的工作. Summary
The seemingly elaborate mechanisms provided by C++ should give you a strong hint about the critical importance placed on initialization and cleanup in the language. As Stroustrup was designing C++, one of the first observations he made about productivity in C was that a significant portion of programming problems are caused by improper initialization of variables. These kinds of bugs are hard to find, and similar issues apply to improper cleanup. Because constructors and destructors allow you to guarantee proper initialization and cleanup (the compiler will not allow an object to be created and destroyed without the proper constructor and destructor calls), you get complete control and safety. Aggregate initialization is included in a similar vein – it prevents you from making typical initialization mistakes with aggregates of built-in types and makes your code more succinct. Safety during coding is a big issue in C++. Initialization and cleanup are an important part of this, but you’ll also see other safety issues as the book progresses.
|
相关阅读 更多 +
