c++ 常用知识

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类型转换

string 转 int

How can I convert a std::string to int?
Easiest way to convert int to string in C++
In C++11 there are some nice new convert functions from std::string to a number type.

So instead of

  • string->int
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    atoi( str.c_str() )

you can use

  • string->int
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    std::stoi( str )

where str is your number as std::string.

There are version for all flavours of numbers:

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long stol(string), float stof(string), double stod(string),...

  • int->string
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    std::string s = std::to_string(42);

see http://en.cppreference.com/w/cpp/string/basic_string/stol

文件操作

Input/output with files

写文件

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// writing on a text file
#include <iostream>
#include <fstream>
using namespace std;

int main () {
  ofstream myfile ("example.txt");
  if (myfile.is_open()) {
    myfile << "This is a line.\n";
    myfile << "This is another line.\n";
    myfile.close();
  }
  else cout << "Unable to open file";
  return 0;
}

读文件

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// reading a text file
#include <iostream>
#include <fstream>
#include <string>
using namespace std;

int main () {
  string line;
  ifstream myfile ("example.txt");
  if (myfile.is_open()) {
    while ( getline (myfile,line) ) {
      cout << line << '\n';
    }
    myfile.close();
  } else {
    cout << "Unable to open file";
  }
  return 0;
}

字符串操作

字符串split

Parse (split) a string in C++ using string delimiter (standard C++)

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std::string s = "scott>=tiger>=mushroom";
std::string delimiter = ">=";

size_t pos = 0;
std::string token;
while ((pos = s.find(delimiter)) != std::string::npos) {
    token = s.substr(0, pos);
    std::cout << token << std::endl;
    s.erase(0, pos + delimiter.length());
}
std::cout << s << std::endl;

// Output:

scott
tiger
mushroom

md5代码

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#include <openssl/md5.h>
#include <string.h>
#include <stdio.h>
#include <iostream>
#include <string>

using namespace std;

int main() {
    MD5_CTX ctx;
    unsigned char outmd[16];
    int i = 0;
    memset(outmd, 0, sizeof(outmd));
    MD5_Init(&ctx);
    MD5_Update(&ctx, "hel", 3);
    MD5_Update(&ctx, "lo", 2);
    MD5_Final(outmd, &ctx);
    for (i = 0; i < 16; i < i++) {
        printf("%02X", outmd[i]);
    }
    printf("\n");
    return 0;
}

进制转换

16进制字符串转为10进制数字

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uint64_t number;
std::stringstream ss;
ss << std::hex << hex_str;
ss >> number;