Rearranged the logic

server/mail_box.cpp

@@ -0,0 +1,93 @@
+#include "mail_box.hpp"
+
+#include <sstream>
+#include <stdexcept>
+
+MailBox::MailBox() {
+	if (!(inLock = SDL_CreateMutex())) {
+		std::ostringstream os;
+		os << "Failed to create a MailBox mutex; ";
+		os << SDL_GetError();
+		throw(std::runtime_error(os.str()));
+	}
+	if (!(outLock = SDL_CreateMutex())) {
+		SDL_DestroyMutex(inLock);
+		std::ostringstream os;
+		os << "Failed to create a MailBox mutex; ";
+		os << SDL_GetError();
+		throw(std::runtime_error(os.str()));
+	}
+}
+
+MailBox::~MailBox() {
+	SDL_DestroyMutex(inLock);
+	SDL_DestroyMutex(outLock);
+}
+
+std::string MailBox::PushIn(std::string msg) {
+	SDL_LockMutex(inLock);
+	input.push_back(msg);
+	SDL_UnlockMutex(inLock);
+	return msg;
+}
+
+std::string MailBox::PeekIn() {
+	std::string ret;
+	SDL_LockMutex(inLock);
+	if (input.size()) {
+		ret = input[0];
+	}
+	else {
+		ret = "";
+	}
+	SDL_UnlockMutex(inLock);
+	return ret;
+}
+
+std::string MailBox::PopIn() {
+	std::string ret;
+	SDL_LockMutex(inLock);
+	if (input.size()) {
+		ret = input[0];
+		input.pop_front();
+	}
+	else {
+		ret = "";
+	}
+	SDL_UnlockMutex(inLock);
+	return ret;
+}
+
+std::string MailBox::PushOut(std::string msg) {
+	SDL_LockMutex(outLock);
+	output.push_back(msg);
+	SDL_UnlockMutex(outLock);
+	return msg;
+}
+
+std::string MailBox::PeekOut() {
+	std::string ret;
+	SDL_LockMutex(outLock);
+	if (output.size()) {
+		ret = output[0];
+	}
+	else {
+		ret = "";
+	}
+	SDL_UnlockMutex(outLock);
+	return ret;
+}
+
+std::string MailBox::PopOut() {
+	std::string ret;
+	SDL_LockMutex(outLock);
+	if (output.size()) {
+		ret = output[0];
+		output.pop_front();
+	}
+	else {
+		ret = "";
+	}
+	SDL_UnlockMutex(outLock);
+	return ret;
+}