C++ string類的c_str()方法小結(jié)

最近研究了一下基于c++的大數(shù)乘法算法, 碰到了string類對(duì)象與c風(fēng)格字符串轉(zhuǎn)換問(wèn)題，有一個(gè)問(wèn)題令我印象深刻，現(xiàn)在沒(méi)有找到具體原因，記錄下來(lái)。代碼環(huán)境是Linux ubuntu，編譯器是g++

覺(jué)得寫(xiě)得比較好的博客羅列如下:

【轉(zhuǎn)】https://www.cnblogs.com/lifexy/p/8642163.html

1.c_str()

c_str()方法返回一個(gè)const char* 類型的指針變量, 該指針變量指向一個(gè)字符數(shù)組, 字符數(shù)組的元素個(gè)數(shù)是string::length() + 1，最后一個(gè)元素為 '\0'

以下為官方說(shuō)法:

const charT* c_str() const;

Returns: A pointer to the initial element of an array of length size() + 1 whose first size() elements equal the corresponding elements of the string controlled by *this and whose last element is a null character specified by charT().

Requires: The program shall not alter any of the values stored in the array. Nor shall the program treat the returned value as a valid pointer value after any subsequent call to a non-const member function of the class basic_string that designates the same object as this.

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2.在實(shí)驗(yàn)中, 不論我在堆上(new)或者在棧上創(chuàng)建一個(gè)string類實(shí)例, 調(diào)用c_str()方法后發(fā)現(xiàn)字符數(shù)組并沒(méi)有隨著實(shí)例的變化而變化，即使實(shí)例銷毀(delete或者自己析構(gòu))，字符數(shù)組也沒(méi)有變化；但是當(dāng)我改變string的原始內(nèi)容時(shí), c_str()方法就會(huì)有所變化, 記錄如下:

當(dāng)string實(shí)例的初始值是"string in stack"時(shí)

root@ubuntu:/lianxi/lianxi_c++/string# g++ test_c_str.cpp 

root@ubuntu:/lianxi/lianxi_c++/string# ./a.out

=====TEST IN STACK=====

aStr.length() = 15

q[0] = 115

q[1] = 116

q[2] = 114

q[3] = 105

q[4] = 110

q[5] = 103

q[6] = 32

q[7] = 105

q[8] = 110

q[9] = 32

q[10] = 115

q[11] = 116

q[12] = 97

q[13] = 99

q[14] = 107

q[15] = 0

&aStr = 0xbf8df7f0, q = 0x93e4014

aStr1:string in stack

q1:string in stack

aStr2:change in stack

q2:change in stack

q3:change in stack

=====TEST OVER=====

root@ubuntu:/lianxi/lianxi_c++/string#

  1 #include <iostream>

  2 #include <string>

  3 #include <string.h>

  4 #include <strings.h>

  5 #include <stdio.h>

  6 

  7 using namespace std;

  8 

  9 /*we use printf in this test instead of cout*/

 10 int main(void)

 11 {

 12     printf("=====TEST IN STACK=====\n");

 13     const char* q = NULL;

 14     /*code block*/

 15     {

 16         string aStr("string in stack");

 17         printf("aStr.length() = %d\n", aStr.length());//15

 18         

 19         q = aStr.c_str();

 20         int j = 0;

 21         for(j = 0; j < aStr.length() + 1; j++)

 22         {

 23             printf("q[%d] = %d\n", j, q[j]);

 24         }   

 25         

 26         printf("&aStr = %p, q = %p\n", &aStr, q);//address are different

 27         

 28         cout << "aStr1:" << aStr << endl;//string in stack

 29         printf("q1:%s\n", q);            //string in stack

 30         

 31         aStr = "change in stack";

 32         cout << "aStr2:" << aStr << endl;//change in stack

 33         printf("q2:%s\n", q);            //change in stack

 34     }   

 35     

 36     printf("q3:%s\n", q);                //change in stack

 37     printf("=====TEST OVER=====\n");

 38     return (0);

 39 }

當(dāng)string實(shí)例的初始值是"abcdefg"時(shí):

root@ubuntu:/lianxi/lianxi_c++/string# g++ test_c_str.cpp 

root@ubuntu:/lianxi/lianxi_c++/string# ./a.out

=====TEST IN STACK=====

aStr.length() = 7

q[0] = 97

q[1] = 98

q[2] = 99

q[3] = 100

q[4] = 101

q[5] = 102

q[6] = 103

q[7] = 0

&aStr = 0xbfbae130, q = 0x9ba4014

aStr1:abcdefg

q1:abcdefg

aStr2:change in stack

q2:abcdefg

q3:abcdefg

=====TEST OVER=====

root@ubuntu:/lianxi/lianxi_c++/string# 

  1 #include <iostream>

  2 #include <string>

  3 #include <string.h>

  4 #include <strings.h>

  5 #include <stdio.h>

  6 

  7 using namespace std;

  8 

  9 /*we use printf in this test instead of cout*/

 10 int main(void)

 11 {

 12     printf("=====TEST IN STACK=====\n");

 13     const char* q = NULL;

 14     /*code block*/

 15     {

 16         //string aStr("string in stack");

 17         string aStr("abcdefg");

 18         printf("aStr.length() = %d\n", aStr.length());//15//7

 19         

 20         q = aStr.c_str();

 21         int j = 0; 

 22         for(j = 0; j < aStr.length() + 1; j++)

 23         {   

 24             printf("q[%d] = %d\n", j, q[j]);

 25         }

 26         

 27         printf("&aStr = %p, q = %p\n", &aStr, q);//address are different

 28         

 29         cout << "aStr1:" << aStr << endl;//string in stack//abcdefg

 30         printf("q1:%s\n", q);            //string in stack//abcdefg

 31         

 32         aStr = "change in stack";

 33         cout << "aStr2:" << aStr << endl;//change in stack

 34         printf("q2:%s\n", q);            //change in stack//abcdefg

 35     }

 36     

 37     printf("q3:%s\n", q);                //change in stack//abcdefg

 38     printf("=====TEST OVER=====\n");

 39     return (0);

 40 }

【注意】:這種機(jī)制目前我并不明確, 估計(jì)是編譯器的某種行為, 但是可以明確看到c_str()方法返回的數(shù)組里面有l(wèi)ength+1個(gè)元素, 且最后一個(gè)元素為'\0'

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原文鏈接：https://blog.csdn.net/liaojunwu/article/details/85269931