C++实现eval

通常的实现方法是表达式树等等，下面看一种不一样的：

// eval.cpp
#include <iostream>
#include <fstream>
using namespace std;

double eval(string s)
{
    fstream fs("eval_calc.cpp", ios::out | ios::trunc);
    fs << "#include <iostream>" << endl;
    fs << "#include <fstream>" << endl;
    fs << "#include <cmath>" << endl;
    fs << "using namespace std;" << endl;
    fs << "int main()" << endl;
    fs << "{" << endl;
    fs << "    ofstream outfile;" << endl;
    fs << "    outfile.open("eval_answer.dat", ios::out | ios::trunc);" << endl;
    fs << "    outfile << " << s << " << endl;" << endl;
    fs << "    outfile.close();" << endl;
    fs << "    return 0;" << endl;
    fs << "}" << endl;
    fs.close();

    system("g++ eval_calc.cpp -o eval_calc.exe");
    system(".\eval_calc.exe");

    ifstream infile;
    infile.open("eval_answer.dat");
    double data;
    infile >> data;
    infile.close();
    system("del eval_calc.cpp");
    system("del eval_calc.exe");
    system("del eval_answer.dat");
    return data;
}

int main()
{
    string e;
    cout << "input an experssion: ";
    cin >> e;
    cout << eval(e) << endl;
    // system("pause");
    return 0;
}

windows下安装gcc（mingw-w64）并配置环境变量，编译运行：

PS C:> g++ eval.cpp -o eval.exe
PS C:> .eval.exe
input an experssion: 1+2*(3-sqrt(4))
3

答案无误！皮这一下很开心！????

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实现原理：新建下面的cpp文件，把要计算的表达式写入，编译执行，就会输出答案到文本中，再读取文本。

// eval_calc.cpp
#include <iostream>
#include <fstream>
#include <cmath>
using namespace std;
int main()
{
    ofstream outfile;
    outfile.open("eval_answer.dat", ios::out | ios::trunc);
    outfile << experssion << endl;
    outfile.close();
    return 0;
}

PS：仅供娱乐，存在漏洞，输入非法字符串会导致编译错误，甚至可能执行危险的命令。

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