Question

来自C11标准

6.5.15条件运算符

语法

conditional-expression:
    logical-OR-expression
    logical-OR-expression ? expression : conditional-expression

逻辑OR表达式是条件表达式，因为它是条件表达式的退化？

6.5.16分配运营商

语法

assignment-expression:
    conditional-expression
    unary-expression  assignment-operator  assignment-expression

这是否意味着条件表达式是一个赋值表达？为什么？

6.6常量表达式

语法
```
constant-expression:    
    conditional-expression
```
常量表达式的语法是否表示常量表达式和常量表达式是相同的概念吗？

感谢。

Answer 1

conditional-expression，assignment-expression和constant-expression是语法中的非终结符。它们的名称不应被视为暗示conditional-expression必须实际涉及条件运算符，或者assignment-expression必须涉及赋值;它们以在优先级层次结构中的位置命名。

特别是，constant-expression扩展为conditional-expression，因为常量不是语法概念。排除需要常量的非常数不是语法的工作;该作业在编译器的其他部分中处理。语法将允许任何需要常量的conditional-expression。

Answer 2

第6.5章写得很差，很难理解。关键是：

C11 6.5 / 3

运算符和操作数的分组由语法表示。

这意味着在标准的任何地方实际上都没有明确指定运算符优先级！相反，我们应该查看每个运算符的语法并找出它。

所有运算符都构建了一个运算符优先级链，因此对于6.5的每个子章节，提到前一组运算符只是为了表明它具有更高的优先级。

从顶部开始，我们有primary-expression postfix-expression ...（6.5.1），优先级最高。在下一组后缀运算符中，将提到前一个组以表明它具有更高的优先级：

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace JamiesWorldGenerator
{
    public partial class WorldGeneratorForm : Form
    {
        World world = null;

        public WorldGeneratorForm()
        {
            InitializeComponent();
        }

        private void runButton_Click(object sender, EventArgs e)
        {
            if (runButton.Text == "Run")
            {
                if (world == null) world = new World(this, 1000);
                world.run();

                runButton.Text = "Stop";
                statusLabel.Text = "Status: Active";
            }
            else if (runButton.Text == "Stop")
            {
                world.pause();

                runButton.Text = "Run";
                statusLabel.Text = "Status: Inactive";
            }
        }

        public void updateTimeElapsed(int i)
        {
            this.daysElapsedLabel.Text = "Days Elapsed: " + i;
        }

        public void updatePersonList(List<Human> list)
        {
            foreach (Human h in list) this.peopleList.Items.Add(h);
            peopleList.DisplayMember = "FullName";
        }
    }
}

这并不意味着主表达式是后缀表达式，而是后缀表达式的语法是namespace JamiesWorldGenerator { partial class WorldGeneratorForm { /// <summary> /// Required designer variable. /// </summary> private System.ComponentModel.IContainer components = null; /// <summary> /// Clean up any resources being used. /// </summary> /// <param name="disposing">true if managed resources should be disposed; otherwise, false.</param> protected override void Dispose(bool disposing) { if (disposing && (components != null)) { components.Dispose(); } base.Dispose(disposing); } #region Windows Form Designer generated code /// <summary> /// Required method for Designer support - do not modify /// the contents of this method with the code editor. /// </summary> private void InitializeComponent() { this.populationLabel = new System.Windows.Forms.Label(); this.runButton = new System.Windows.Forms.Button(); this.statusLabel = new System.Windows.Forms.Label(); this.panel1 = new System.Windows.Forms.Panel(); this.countriesList = new System.Windows.Forms.ListBox(); this.panel2 = new System.Windows.Forms.Panel(); this.peopleList = new System.Windows.Forms.ListBox(); this.panel3 = new System.Windows.Forms.Panel(); this.worldNews = new System.Windows.Forms.ListBox(); this.button1 = new System.Windows.Forms.Button(); this.button2 = new System.Windows.Forms.Button(); this.daysElapsedLabel = new System.Windows.Forms.Label(); this.panel1.SuspendLayout(); this.panel2.SuspendLayout(); this.panel3.SuspendLayout(); this.SuspendLayout(); // // populationLabel // this.populationLabel.AutoSize = true; this.populationLabel.Location = new System.Drawing.Point(9, 9); this.populationLabel.Name = "populationLabel"; this.populationLabel.Size = new System.Drawing.Size(94, 13); this.populationLabel.TabIndex = 0; this.populationLabel.Text = "World Population: "; // // runButton // this.runButton.Location = new System.Drawing.Point(8, 320); this.runButton.Name = "runButton"; this.runButton.Size = new System.Drawing.Size(75, 23); this.runButton.TabIndex = 1; this.runButton.Text = "Run"; this.runButton.UseVisualStyleBackColor = true; this.runButton.Click += new System.EventHandler(this.runButton_Click); // // statusLabel // this.statusLabel.AutoSize = true; this.statusLabel.Location = new System.Drawing.Point(89, 325); this.statusLabel.Name = "statusLabel"; this.statusLabel.Size = new System.Drawing.Size(81, 13); this.statusLabel.TabIndex = 2; this.statusLabel.Text = "Status: Inactive"; // // panel1 // this.panel1.Controls.Add(this.countriesList); this.panel1.Location = new System.Drawing.Point(12, 25); this.panel1.Name = "panel1"; this.panel1.Size = new System.Drawing.Size(197, 187); this.panel1.TabIndex = 3; // // countriesList // this.countriesList.Dock = System.Windows.Forms.DockStyle.Fill; this.countriesList.FormattingEnabled = true; this.countriesList.Location = new System.Drawing.Point(0, 0); this.countriesList.Name = "countriesList"; this.countriesList.Size = new System.Drawing.Size(197, 187); this.countriesList.TabIndex = 1; // // panel2 // this.panel2.Controls.Add(this.peopleList); this.panel2.Location = new System.Drawing.Point(215, 25); this.panel2.Name = "panel2"; this.panel2.Size = new System.Drawing.Size(197, 187); this.panel2.TabIndex = 4; // // peopleList // this.peopleList.Dock = System.Windows.Forms.DockStyle.Fill; this.peopleList.FormattingEnabled = true; this.peopleList.Location = new System.Drawing.Point(0, 0); this.peopleList.Name = "peopleList"; this.peopleList.Size = new System.Drawing.Size(197, 187); this.peopleList.TabIndex = 0; // // panel3 // this.panel3.Controls.Add(this.worldNews); this.panel3.Location = new System.Drawing.Point(12, 219); this.panel3.Name = "panel3"; this.panel3.Size = new System.Drawing.Size(400, 95); this.panel3.TabIndex = 5; // // worldNews // this.worldNews.Dock = System.Windows.Forms.DockStyle.Fill; this.worldNews.FormattingEnabled = true; this.worldNews.Location = new System.Drawing.Point(0, 0); this.worldNews.Name = "worldNews"; this.worldNews.Size = new System.Drawing.Size(400, 95); this.worldNews.TabIndex = 0; // // button1 // this.button1.Location = new System.Drawing.Point(336, 320); this.button1.Name = "button1"; this.button1.Size = new System.Drawing.Size(75, 23); this.button1.TabIndex = 6; this.button1.Text = "Load World"; this.button1.UseVisualStyleBackColor = true; // // button2 // this.button2.Location = new System.Drawing.Point(255, 320); this.button2.Name = "button2"; this.button2.Size = new System.Drawing.Size(75, 23); this.button2.TabIndex = 7; this.button2.Text = "Save World"; this.button2.UseVisualStyleBackColor = true; // // daysElapsedLabel // this.daysElapsedLabel.AutoSize = true; this.daysElapsedLabel.Location = new System.Drawing.Point(212, 9); this.daysElapsedLabel.Name = "daysElapsedLabel"; this.daysElapsedLabel.Size = new System.Drawing.Size(84, 13); this.daysElapsedLabel.TabIndex = 8; this.daysElapsedLabel.Text = "Days Elapsed: 0"; // // WorldGeneratorForm // this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 13F); this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font; this.ClientSize = new System.Drawing.Size(423, 351); this.Controls.Add(this.daysElapsedLabel); this.Controls.Add(this.button2); this.Controls.Add(this.button1); this.Controls.Add(this.panel3); this.Controls.Add(this.panel2); this.Controls.Add(this.panel1); this.Controls.Add(this.statusLabel); this.Controls.Add(this.runButton); this.Controls.Add(this.populationLabel); this.Name = "WorldGeneratorForm"; this.StartPosition = System.Windows.Forms.FormStartPosition.CenterScreen; this.Text = "Jamie\'s World Generator"; this.panel1.ResumeLayout(false); this.panel2.ResumeLayout(false); this.panel3.ResumeLayout(false); this.ResumeLayout(false); this.PerformLayout(); } #endregion private System.Windows.Forms.Label populationLabel; private System.Windows.Forms.Button runButton; private System.Windows.Forms.Label statusLabel; private System.Windows.Forms.Panel panel1; private System.Windows.Forms.Panel panel2; private System.Windows.Forms.ListBox countriesList; private System.Windows.Forms.ListBox peopleList; private System.Windows.Forms.Panel panel3; private System.Windows.Forms.ListBox worldNews; private System.Windows.Forms.Button button1; private System.Windows.Forms.Button button2; private System.Windows.Forms.Label daysElapsedLabel; } }

整个6.5章节都是这样的。它可能听起来很愚蠢而且仅仅是因为它皇家愚蠢。

注85）尝试提供一些帮助：

语法指定运算符在表达式求值中的优先级，这是相同的作为本条款主要子条款的顺序，首先是最高优先级。

如何理解条件，赋值和常量表达式的语法？

2 个答案: