系列入口：编程实战：类C语法的编译型脚本解释器（九）编译语句

         本文介绍表达式的编译。

一、代码概览

         表达式的编译就是不断获取下一个标识符，直到遇到不属于表达式的东西。

        完整代码如下：

		Expression* GetExpression(CTokens& tokens, T_VARIABLE_S& vars, Expression* pExpression, size_t& pos, char const* endch, bool half = false)
		{
			size_t start_pos = pos;
			size_t op_pos = 0;
			Token* pToken;
			Expression* pOperand = NULL;//下一个操作数
			Token* pOperator = NULL;//指向下一个操作符

			while (true)
			{
				if (NULL == pOperand && NULL == pOperator)
				{
					tokens.MoveCurrentToken(pToken, pos);
					if (tokens.IsPosNotToken(pos))
					{
						return pExpression;
					}

					if (isExpressionEnd(tokens, pos, endch))
					{//空语句
						if (NULL == pExpression)
						{
							pExpression = NewExpression();
							pExpression->source_start = tokens.TokenStart(pos);
							pExpression->source_end = tokens.TokenStart(pos);
							if (NULL == pExpression)throw "内存不足";
						}
						return pExpression;
					}
					//取下一个操作数，可能没有（当下一个标记是操作符）
					if (NULL != pOperator)cout << pOperator->text << endl;
					GetOperand(tokens, vars, pOperand, pos);
					//取下一个操作符
					if (GetOperator(tokens, pOperator, pos))op_pos = pos - 1;
				}

				if (NULL == pOperand && NULL == pOperator)
				{//什么都没取到，结束
					return pExpression;
				}

				if (NULL == pExpression)
				{//新建表达式
					if (NULL == pOperator || isExpressionEnd(tokens, pos - 1, endch))
					{
						if (isExpressionEnd(tokens, pos - 1, endch))--pos;
						else
						{
							string str;
							if (!isExpressionEnd(tokens, pos, endch))
							{
								cout << tokens.m_tokens[pos - 1].text << " " << endch << endl;
								cout << pOperand->ToString(m_source, vars) << endl;
								CException::Throw(__FILE__, __LINE__, m_source, tokens.TokenStart(pos), "期待操作符");
							}
						}
						pExpression = pOperand;
						return pExpression;
					}
					else
					{
						pExpression = NewExpression();
						if (NULL == pExpression)throw "内存不足";
						pExpression->source_start = tokens.TokenStart(start_pos);
						pExpression->source_end = tokens.TokenStart(op_pos);
						pExpression->type = Expression::OPERATION;
						pExpression->op = pOperator->text;
						if (NULL != pOperand)pExpression->AddLeftOperand(*pOperand);
					}
					pOperand = NULL;
					pOperator = NULL;
				}
				else
				{//追加表达式
					if (NULL == pOperator || isExpressionEnd(tokens, pos - 1, endch))
					{//操作符为空
						if (isExpressionEnd(tokens, pos - 1, endch))--pos;
						pExpression->AddRightOperand(*pOperand);
						pExpression->source_end = tokens.TokenStart(pos);
						pOperand = NULL;
						return pExpression;
					}
					else
					{
						long level1, level2;
						if (NULL == pExpression->pLeftOperand())level1 = 2;//单目算符
						else if (!tokens.GetOperatorLevel(pExpression->op.c_str(), level1))Throw(__FILE__, __LINE__, pExpression->op + " 操作符优先级未知或此处不应该出现此操作符");
						if (NULL == pOperand)level2 = 2;//单目算符
						else if (!tokens.GetOperatorLevel(pOperator->text.c_str(), level2))Throw(__FILE__, __LINE__, pOperator->text + " 操作符优先级未知或此处不应该出现此操作符");
						if (level1 < level2 || level1 == level2 && tokens.IsOperatorLeftFirst(level1))
						{
							pExpression->AddRightOperand(*pOperand);
							pExpression->source_end = tokens.TokenStart(pos - 1);
							if (half)
							{
								--pos;//退回最后一个操作符
								return pExpression;
							}
							Expression* tmp = NewExpression();
							if (NULL == tmp)throw "内存不足";
							tmp->type = Expression::OPERATION;
							tmp->source_start = pExpression->source_start;
							tmp->source_end = op_pos;
							tmp->op = pOperator->text;
							tmp->AddLeftOperand(*pExpression);
							pExpression = tmp;
							pOperand = NULL;
							pOperator = NULL;
						}
						else
						{
							Expression* tmp = NewExpression();
							if (NULL == tmp)throw "内存不足";
							tmp->type = Expression::OPERATION;
							if (NULL == pOperand)tmp->source_start = tokens.TokenStart(op_pos);
							else tmp->source_start = pOperand->source_start;
							tmp->source_end = tokens.TokenStart(op_pos);
							tmp->op = pOperator->text;
							tmp->AddLeftOperand(*pOperand);
							pOperand = NULL;
							pOperator = NULL;

							pOperand = GetExpression(tokens, vars, tmp, pos, endch, true);
							if (GetOperator(tokens, pOperator, pos))op_pos = pos - 1;
						}
					}
				}
			}
			//return pExpression;
		}

        参数char const* endch指出期待什么样的结尾，分为“}”和“)”两种情况。

（我还没写完）