浅谈const-白红宇

浅谈const

阅读量：2431 次

发布时间：2019-05-10

本文共 4692 字，大约阅读时间需要 15 分钟。

讲讲const的基本用法

---------------------------------------------------------------

1.声明常量

const int a=0;

a=5; //error

2.修饰函数行参，保证行参在函数中不被修改

void fn(int i)

{

i=5;//error

}

3.修饰成员函数,使成员函数只能调用成员变量，且使用时不被改变

class A

{

int i;

int f()const{return i;}

}

---------------------------------------------------------------

const declaration

member-function const

When modifying a data declaration, the const keyword specifies that the object or variable is not modifiable. When following a member function's parameter list, the const keyword specifies that the function doesn't modify the object for which it is invoked.

Constant Values

The const keyword specifies that a variable's value is constant and tells the compiler to prevent the programmer from modifying it.

const int i = 5;

i = 10; // Error

i++; // Error

In C++, you can use the const keyword instead of the #define preprocessor directive to define constant values. Values defined with const are subject to type checking, and can be used in place of constant expressions. In C++, you can specify the size of an array with a const variable as follows:

const int maxarray = 255;

char store_char[maxarray]; // Legal in C++; illegal in C

In C, constant values default to external linkage, so they can appear only in source files. In C++, constant values default to internal linkage, which allows them to appear in header files.

The const keyword can also be used in pointer declarations.

char *const aptr = mybuf; // Constant pointer

*aptr = 'a'; // Legal

aptr = yourbuf; // Error

A pointer to a variable declared as const can be assigned only to a pointer that is also declared as const.

const char *bptr = mybuf; // Pointer to constant data

*bptr = 'a'; // Error

bptr = yourbuf; // Legal

You can use pointers to constant data as function parameters to prevent the function from modifying a parameter passed through a pointer.

You can call constant member functions only for a constant object. This ensures that the object is never modified.

birthday.getMonth(); // Okay

birthday.setMonth( 4 ); // Error

You can call either constant or nonconstant member functions for a nonconstant object. You can also overload a member function using the const keyword; this allows a different version of the function to be called for constant and nonconstant objects.

You cannot declare constructors or destructors with the const keyword.

Constant Member Functions

C++ Specific

Declaring a member function with the const keyword specifies that the function is a "read-only" function that does not modify the object for which it is called.

To declare a constant member function, place the const keyword after the closing parenthesis of the argument list. The const keyword is required in both the declaration and the definition. A constant member function cannot modify any data members or call any member functions that aren't constant.

END C++ Specific

Example

// Example of a constant member function

class Date

{

public:

Date( int mn, int dy, int yr );

int getMonth() const; // A read-only function

void setMonth( int mn ); // A write function;

// cannot be const

private:

int month;