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dfe1c933b3
- const README: rewrite the space-saving claim per issue #5 follow-ups (constant-expression const vs #define immediates; which const qualify as constant expressions) - const README: fix the class-section diagnosis per issue #338 (overload resolution picks the const overload; the restriction only surfaces once that overload is removed) - sizeof README: virtual-inheritance result is compiler-dependent (GCC merges vbptr+vfptr -> 16, MSVC keeps them separate -> 32) #246 - static README: new section on internal linkage / access restriction with anonymous-namespace comparison #142 - 10-day practice: full problem list table + fixed day links #136 - new design_pattern/singleton/singleton_default.hpp: reusable thread-safe CRTP singleton template, compile+concurrency tested #303
100 lines
1.8 KiB
Markdown
100 lines
1.8 KiB
Markdown
> 本文档由 [C++那些事网站](https://light-city.github.io/stories_things/cpp-tools/cpp-string-to-int/) 同步生成，网站版本含多语言与更新排版。
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# C++ string 转 int
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个人公众号：
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首先提出解决方案：
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- atoi
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- strtol
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- stoi
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这几个有什么不同呢？下面测试对比。
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> C语言风格函数
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atoi与strtol对比：
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```cpp
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string str = "16s";
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int a = atoi(str.c_str());
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int b = strtol(str.c_str(), nullptr, 10);
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```
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输出：
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```
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atoi的结果为:16
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strtol的结果为:16
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```
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这两个函数都是从字符串开始寻找数字或者正负号或者小数点,遇到非法字符终止。
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所以到上述s字符就不输出了，提前结束，也就是说当你的字符串不是数字的时候，或者小数点等非数字，不会报异常！直接输出0.
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例如：
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```cpp
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string str = "asdsa";
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int a = atoi(str.c_str());
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int b = strtol(str.c_str(), nullptr, 10);
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```
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输出：
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```
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0
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0
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```
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strtol相比与atoi来说，支持多种进制转换,例如8进制等
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例如：
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```
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int b = strtol(str.c_str(), nullptr, 8);
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```
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> C++风格
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在C++中可以使用stoi来转int，这个函数相比于前两个一个最大特点是：异常！
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我们知道C++相比于C语言多了异常，这也是这个函数在C++中具有的最显著功能。
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例如：
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```cpp
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string str1 = "asq,";
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// int c = stoi(str1); // 报异常
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string str2 = "12312";
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int c = stoi(str2); // ok
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cout << c << endl;
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```
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异常如下：
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```
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terminate called after throwing an instance of 'std::invalid_argument'
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what(): stoi
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```
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> 自定义
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也就是自己写呗，如下：
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```cpp
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int stringToInt(const string &s) {
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int v;
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stringstream ss;
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ss << s;
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ss >> v;
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return v;
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}
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int main() {
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int i = stringToInt("2.3");
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cout<<i<<endl;
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}
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``` |