feat: 用Boost.DLL从指定目录用加载动态链接库！

.gitignore

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+# clangd的缓存目录
+.cache
+# 为clangd创建的compile_commands.json符号链接
+compile_commands.json

README.md

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+# loader
+
+将`/usr/lib/loader`和`/usr/local/lib/loader`里的动态链接库作为插件加载！
+
+可以通过`LOADER_PLUGIN_DIRS`覆盖默认插件目录！
+Linux下用`:`分割路径，Windows下用`;`分割路径。
+
+为了方便测试，用
+[just](https://github.com/casey/just)
+封装了meson！
+
+```console
+$ meson setup builddir
+$ just install
+$ /usr/local/bin/loader
+```

include/interface.hpp

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+#pragma once
+
+#include <string>
+
+#include <boost/config.hpp>
+
+class BOOST_SYMBOL_VISIBLE Interface {
+public:
+    virtual const std::string name() const = 0;
+    virtual ~Interface() {}
+};

justfile

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+build:
+    meson compile -C builddir
+
+install: build
+    meson install -C builddir

main.cpp

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+#include <cstdlib>
+
+#include <string>
+#include <vector>
+#include <iostream>
+
+#include <boost/algorithm/string.hpp>
+#include <boost/filesystem.hpp>
+#include <boost/dll/import.hpp>
+
+#include "interface.hpp"
+
+using std::string;
+using std::vector;
+
+namespace fs = boost::filesystem;
+namespace dll = boost::dll;
+
+const vector<string> &get_plugin_dirs() {
+    static vector<string> result;
+    if (!result.empty()) {
+        return result;
+    }
+
+#ifdef _WIN32
+    const string delimiter = ";";
+#else
+    const string delimiter = ":";
+#endif
+    const char *plugin_dirs = std::getenv("LOADER_PLUGIN_DIRS");
+    if (plugin_dirs != nullptr && *plugin_dirs != '\0') {
+        boost::split(result, plugin_dirs, boost::is_any_of(delimiter));
+    }
+
+    if (result.empty()) {
+        result.push_back("/usr/lib/loader");
+        result.push_back("/usr/local/lib/loader");
+    }
+
+    return result;
+}
+
+int main() {
+    const vector<string> &plugin_dirs = get_plugin_dirs();
+    for (const auto &plugin_dir : plugin_dirs) {
+        fs::path plugin_path(plugin_dir);
+
+        if (!fs::is_directory(plugin_path)) {
+            continue;
+        }
+        for (fs::recursive_directory_iterator dir(plugin_path), end; dir != end; ++dir) {
+            boost::shared_ptr<Interface> plugin;
+            plugin = dll::import_symbol<Interface>(
+                dir->path(),
+                "plugin",
+                dll::load_mode::rtld_lazy
+            );
+            std::cout << "loading plugin: " << plugin->name() << std::endl;
+        }
+    }
+}

meson.build

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+project('loader', 'cpp',
+  default_options: ['cpp_std=c++20'])
+
+inc = include_directories('include')
+plugin_dep = declare_dependency(include_directories : inc)
+
+boost_dep = dependency('boost', modules : ['filesystem'])
+executable('loader', 'main.cpp',
+  include_directories : inc,
+  dependencies : boost_dep,
+  install : true)
+
+subdir('plugins')

plugins/command/meson.build

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+command = shared_library('command', 'plugin.cpp',
+  dependencies : plugin_dep,
+  install : true,
+  install_dir : 'lib/loader')

plugins/command/plugin.cpp

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+#include "plugin.hpp"
+
+#include <iostream>
+#include <string>
+
+using std::string;
+
+namespace loader {
+
+const string Command::name() const {
+    return "command";
+}
+Command::~Command() {
+    std::cout << "destructing command" << std::endl;
+}
+
+}

plugins/command/plugin.hpp

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+#pragma once
+
+#include <string>
+
+#include "interface.hpp"
+
+namespace loader {
+
+class Command : public Interface {
+public:
+    const std::string name() const;
+    ~Command();
+};
+
+extern "C" BOOST_SYMBOL_EXPORT Command plugin;
+Command plugin;
+
+}

plugins/meson.build

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+subdir('command')