站在巨人肩上的思考[连载] （8）

 

5.2 数组

5.2.2 字符串文字量

         类似”this is a string.”这样一个字符串文字量，它是一个常量。正如代码中一个显式的数字0xff08一样，其值不可以被更改。如想修改，可以通过复制给一个变量来实现：

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int a = 0xff08;

a = 0xff18;

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         对于字符串文字量则有一点要注意：

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char* cp = “this is a string”;       //危险！赋值给一个普通字符指针；

cp[4] = ‘e’;                     //未定义行为！不能给常量赋值；

char p[] = “this is a string”;        //可以；

p[4] = ‘e’;                      //正确执行；

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         可以看出，用char *来引用字符串文字量是一个十分危险的地方。在上例中cp[4]=’e’;可以通过编译，但是在执行期间产生了一个Segmentation Fault。也就是说编译器并不能帮助程序员发现这类错误。

我个人是这样认为的：字符串文字量是一中很底层的数据，编译器把他放进汇编语言中的数据段；普通的char*指针也是一种很底层的指针，他可以指向内存空间中的任何地址；char* cp = “this is a string”;仅仅在栈空间生成一个指针，然后把他指向位于数据段的”this is a string”，初始化并不会导致错误。但是利用cp[4]更改数据段中的数据内容将会导致段错误，数据段是只读的。而利用char p[]形式的时候，意味着将在栈空间中生成一个字符数组，然后用”this is a string”来进行初始化，而后p指向这个栈空间中字符数组的首地址。此时p[4]=’e’;修改的是栈空间中的值，而不是数据段中的值。

但是p是什么类型呢？是char*么？他的大小是多少？是1么？为了使指针和数组首地址之间的区别更具普遍意义，我们用int类型来测试一下：

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char p[] = "hello";

char* ptr = p;

cout << sizeof(p) << ":" << sizeof(*p) << endl;

cout << p << ":" << *p << endl;

cout << sizeof(ptr) << ":" << sizeof(*ptr) << endl;

cout << ptr << ":" << *ptr << endl;

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这段代码在我的电脑上的输出是：

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[/home/gavin/test]# ./test

6:1

hello:h

4:1

hello:h

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ptr是真正的char*类型，其sizeof是4，但是p的sizeof却是6，也就是”hello”加一个‘\0’的长度。除此之外，ptr和p表现都一样。但我们最好认为p是字符数组类型，而ptr才是字符指针类型，因为他们的行为上存在差异。很多教师、教材都说：“数组名就是指向数组首地址的指针。”这其实并不完全正确。

再仔细看看，我们cout<<p和cout<<ptr的期望输出应该是p和ptr分别表示的地址。但事实上却是数组的内容。为了明显区分，再看下面这段代码：

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int arr[4] = {0};

int* pa = arr;

cout << sizeof(arr) << ":" << sizeof(*arr) << endl;

cout << arr << ":" << *arr << endl;

cout << sizeof(pa) << ":" << sizeof(*pa) << endl;

cout << pa << ":" << *pa << endl;

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在我的电脑上输出为：

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[/home/gavin/test]# ./test

16:4

0xbfc26e5c:0

4:4

0xbfc26e5c:0

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sizeof的输出刚才已经说过。这里可以看出，int数组名和char数组名的行为也不一样。事实上，cha数组名与标准类型数组名的行为都不一样。这也就是为什么C风格字符串为许多人诟病的地方。

C++在这方面提供了很好的改进：用一种更上层的类型string来代替C风格字符串。并强烈建议程序员在程序中尽量使用string来代替char *。这里是你需要做出选择的地方：

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string str1("1");

string str2("1234567");

string str3("1234567890123456789012345678901234567890");

/*

char str1[] = "1";

char str2[] = "1234567";

char str3[] = "1234567890123456789012345678901234567890";

*/

cout << sizeof(str1) << endl;

cout << sizeof(str2) << endl;

cout << sizeof(str3) << endl;

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当利用C风格字符串的输出为：

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[/home/gavin/test]# ./test

2

8

41

[/home/gavin/test]# ls -l

-rwxrwxr-x 1 863 863    6559 Dec  3 14:29 test

-rw-rw-r-- 1 863 863     391 Dec  3 14:29 test.cpp

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而当使用string时：

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[/home/gavin/test]# ./test

4

4

4

[/home/gavin/test]# ls -l

-rwxrwxr-x 1 863 863    7578 Dec  3 14:30 test

-rw-rw-r-- 1 863 863     390 Dec  3 14:30 test.cpp

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我们也许可以得到如下结论：sting比char[]需要更少的运行时内存空间，但是char[]却比string占用更少的磁盘空间。你可以自己选择向左还是向右走。不过相比string的易用性、安全性、以及诸多的成员函数带来的实用功能来说，我个人并不认为char[]节省的磁盘空间是很突出的优点。