c++ - 如何使用 Boost 扩展 .ini 文件中的环境变量

Question

我有一个像这样的 INI 文件

[Section1]
Value1 = /home/%USER%/Desktop
Value2 = /home/%USER%/%SOME_ENV%/Test

并想使用 Boost 解析它。我尝试像这样使用 Boost property_tree

boost::property_tree::ptree pt;
boost::property_tree::ini_parser::read_ini("config.ini", pt);

std::cout << pt.get<std::string>("Section1.Value1") << std::endl;
std::cout << pt.get<std::string>("Section1.Value2") << std::endl;

但是并没有展开环境变量。输出看起来像

/home/%USER%/Desktop
/home/%USER%/%SOME_ENV%/Test

我期待的是这样的

/home/Maverick/Desktop
/home/Maverick/Doc/Test

我不确定是否可以使用 boost property_tree。

如果有任何使用 boost 解析此类文件的提示，我将不胜感激。

Answer 1

这是另一种方式，使用旧工艺:

• 不需要Spirit，也不需要Boost
• 不将接口(interface)硬连接到 std::string(而是允许输入迭代器 和输出迭代器 的任意组合)
• 将 %% 作为单个 % “正确”处理 ¹

本质:

#include <string>
#include <algorithm>

static std::string safe_getenv(std::string const& macro) {
    auto var = getenv(macro.c_str());
    return var? var : macro;
}

template <typename It, typename Out>
Out expand_env(It f, It l, Out o)
{
    bool in_var = false;
    std::string accum;
    while (f!=l)
    {
        switch(auto ch = *f++)
        {
            case '%':
                if (in_var || (*f!='%'))
                {
                    in_var = !in_var;
                    if (in_var) 
                        accum.clear();
                    else
                    {
                        accum = safe_getenv(accum);
                        o = std::copy(begin(accum), end(accum), o);
                    }
                    break;
                } else 
                    ++f; // %% -> %
            default:
                if (in_var)
                    accum += ch;
                else
                    *o++ = ch;
        }
    }
    return o;
}

#include <iterator>

std::string expand_env(std::string const& input)
{
    std::string result;
    expand_env(begin(input), end(input), std::back_inserter(result));
    return result;
}

#include <iostream>
#include <sstream>
#include <list>

int main()
{
    // same use case as first answer, show `%%` escape
    std::cout << "'" << expand_env("Greeti%%ng is %HOME% world!")  << "'\n";

    // can be done streaming, to any container
    std::istringstream iss("Greeti%%ng is %HOME% world!");
    std::list<char> some_target;

    std::istreambuf_iterator<char> f(iss), l;
    expand_env(f, l, std::back_inserter(some_target));
    std::cout << "Streaming results: '" << std::string(begin(some_target), end(some_target)) << "'\n";

    // some more edge cases uses to validate the algorithm (note `%%` doesn't
    // act as escape if the first ends a 'pending' variable)
    std::cout << "'" << expand_env("")                           << "'\n";
    std::cout << "'" << expand_env("%HOME%")                     << "'\n";
    std::cout << "'" << expand_env(" %HOME%")                    << "'\n";
    std::cout << "'" << expand_env("%HOME% ")                    << "'\n";
    std::cout << "'" << expand_env("%HOME%%HOME%")               << "'\n";
    std::cout << "'" << expand_env(" %HOME%%HOME% ")             << "'\n";
    std::cout << "'" << expand_env(" %HOME% %HOME% ")            << "'\n";
}

在我的盒子上打印:

'Greeti%ng is /home/sehe world!'
Streaming results: 'Greeti%ng is /home/sehe world!'
''
'/home/sehe'
' /home/sehe'
'/home/sehe '
'/home/sehe/home/sehe'
' /home/sehe/home/sehe '
' /home/sehe /home/sehe '

---

¹ 当然，“适当”是主观的。至少，我认为这是

• 会有用(否则您将如何配置一个合法包含 % 的值？)
• cmd.exe 在 Windows 上是如何工作的