Boost【2】ScopeExit

需求

  • 资源有很多种,每种都封装一套,还是挺累的!对于比较少使用或者一个程序很可能只会用一次的资源,我们不想封装!

  • Golang 的 defer 真香!

解决

利用 RAII 特性,封装个 ScopeGuard!或者直接用 Boost.ScopeExit。

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// umu/scope_exit.hpp
#pragma once

#include <functional>

// C++11

namespace umu {
class ScopeGuard {
public:
explicit ScopeGuard(std::function<void()> on_exit_scope)
: on_exit_scope_(on_exit_scope), dismissed_(false) {}

~ScopeGuard() noexcept {
if (!dismissed_) {
on_exit_scope_();
}
}

void Dismiss() { dismissed_ = true; }

private:
std::function<void()> on_exit_scope_;
bool dismissed_;

// noncopyable
ScopeGuard(ScopeGuard const&) = delete;
ScopeGuard& operator=(ScopeGuard const&) = delete;
};
} // end of namespace umu

#define SCOPEGUARD_LINENAME_CAT(name, line) name##line
#define SCOPEGUARD_LINENAME(name, line) SCOPEGUARD_LINENAME_CAT(name, line)
#define ON_SCOPE_EXIT(callback) \
umu::ScopeGuard SCOPEGUARD_LINENAME(EXIT, __LINE__)(callback)

范例

Windows 上使用 socket 必须先调用 WSAStartup 初始化 WinSock 环境。

常规封装版

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#include <winsock2.h>

#pragma comment(lib, "ws2_32.lib")

class WinSock {
public:
WinSock() : error_code_(WSAEFAULT) {}

int Initialize(WORD version_requested = WINSOCK_VERSION) {
assert(WSAEFAULT == error_code_);
return error_code_ = ::WSAStartup(version_requested, &wsa_data_);
}

bool GetWsaData(WSADATA& wsa_data) {
if (NO_ERROR == error_code_) {
wsa_data = wsa_data_;
return true;
}
return false;
}

int GetErrorCode() { return error_code_; }

~WinSock() {
if (NO_ERROR == error_code_) {
::WSACleanup();
}
}

private:
int error_code_;
WSADATA wsa_data_;
}

int main(int argc, char* argv[]) {
WinSock winsock;
int error_code = winsock.Initialize();
if (NO_ERROR != error_code) {
std::cerr << "Initialize() failed with " << error_code << '\n';
return error_code;
}

// test codes
SOCKET s = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
if (INVALID_SOCKET != s) {
closesocket(s);
std::cout << "OK\n";
} else {
error_code = WSAGetLastError();
if (WSANOTINITIALISED == error_code) {
std::cerr << "WSANOTINITIALISED\n";
} else {
std::cerr << "socket() failed with " << error_code << '\n';
}
}

return error_code;
}

Boost.ScopeExit 版

现在是 2020 年 9 月,建议使用 cpp17,所以抛弃 BOOST_SCOPE_EXIT + BOOST_SCOPE_EXIT_END,使用 cpp11 的 BOOST_SCOPE_EXIT_ALL。

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#include <winsock2.h>

#include <boost/scope_exit.hpp>

#pragma comment(lib, "ws2_32.lib")

int main(int argc, char* argv[]) {
WSADATA wsa_data = {};
int error_code = ::WSAStartup(WINSOCK_VERSION, &wsa_data);
if (NO_ERROR != error_code) {
std::cerr << "WSAStartup() failed with " << error_code << '\n';
return error_code;
}
// 类似 Golang 的 defer
BOOST_SCOPE_EXIT_ALL(&) { ::WSACleanup(); };

// test codes
SOCKET s = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
if (INVALID_SOCKET != s) {
closesocket(s);
std::cout << "OK\n";
} else {
error_code = WSAGetLastError();
if (WSANOTINITIALISED == error_code) {
std::cerr << "WSANOTINITIALISED\n";
} else {
std::cerr << "socket() failed with " << error_code << '\n';
}
}

return error_code;
}

参考

https://www.boost.org/doc/libs/1_74_0/libs/scope_exit/doc/html/BOOST_SCOPE_EXIT_ALL.html