关于c_str()的用法
引入
首先来看一段代码：
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| #include <string>
#include <fstream>
#include <sstream>
#include <iostream>
#include <string.h>
std::string readFile(const char *path)
{
std::string str;
std::ifstream file;
std::stringstream stream;
file.exceptions(std::ifstream::failbit | std::ifstream::badbit);
try
{
file.open(path);
stream << file.rdbuf();
str = stream.str();
file.close();
}
catch (const std::exception &e)
{
std::cerr << e.what() << '\n';
}
return str;
}
int main()
{
freopen("output.txt", "w", stdout);
const char *c = readFile("a.txt").c_str();
printf("%s\n", c);
const char *d = readFile("b.txt").c_str();
printf("%s\n", c);
return 0;
}
|
a.txt:
a.txt
b.txt:
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| It never rains but it pours.
|
期待的结果自然是两遍 a.txt 中的内容，而当打开 output.txt 时，奇怪的事情发生了：
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| Every dog has it's day.
It never rains but it pours.
|
分析
在 c_str() 定义的注释中可以找到这样一句警告：
!!! warning
Do not modify or dire things may happen.
发生上面问题的原因在于 c_str() 返回的只是一个临时的指针，下一次操作正好用了相同的地址，使得内容发生了改变。
解决
将字符串复制出来即可：
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| int main()
{
freopen("output.txt", "w", stdout);
std::string tmp;
tmp = readFile("a.txt");
char *c = new char[tmp.length()];
strcpy(c, tmp.c_str());
printf("%s\n", c);
tmp = readFile("b.txt");
char *d = new char[tmp.length()];
strcpy(d, tmp.c_str());
printf("%s\n", c);
return 0;
}
|
输出：
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2
| Every dog has it's day.
Every dog has it's day.
|