C++Basic Introduction

1.C++First acquaintance

1.1.Operating Conventions for This Chapter

本章的代码示例默认在 C++开发环境搭建与测试 下载并跑通的 CMake 模板中运行。

Starting from this chapter, for the preliminary grammar exercises, only modify this file:

src/main.cpp

For each example, you can copy the code into src/main.cpp and then rebuild and run the template program.

For now, do not modify these files and directories:

1. CMakeLists.txt
2. src/CMakeLists.txt
3. src/lib1/
4. src/lib2/

There is also no need to manually write g++ main.cpp. We will continue to use the CMake template that was already working in the previous chapter to compile the program.

Common commands:

Linux：

cmake --build --preset linux-debug
./build/linux-debug/src/cmake_template

Windows，PowerShell：

cmake --build --preset windows-debug
./build/windows-debug/bin/cmake_template.exe

If you have not yet completed the engineering environment setup from the previous chapter, please first see the section starting from C++Development Environment and the First Project; this tutorial has consistently used the same CMake template. At this point, your project should still roughly have this structure.

1.2.The firstC++program

Open the following in the project:

src/main.cpp

Replace the content with:

#include <iostream>

using namespace std;

int main()
{
    // 程序从 main 函数开始执行，下面的语句会按顺序运行。
	cout << "Hello world" << endl;

	// 返回 0 表示程序正常结束。
	return 0;
}

Operation result:

Hello world

1.2.1.Build and run the program

Execute in the project root directory: (or directly use the graphical interface compilation discussed in the previous section, and below I will also teach you again)

Linux：

cmake --build --preset linux-debug
./build/linux-debug/src/cmake_template

Windows，PowerShell：

cmake --build --preset windows-debug
./build/windows-debug/bin/cmake_template.exe

Graphical steps:

Configured in build+debug mode:

Click build in the lower left corner.

Run

Terminal output:

Hello world

1.3.main function parameters

main Functions do not have to take parameters. The two most common forms are:

int main()
{
	// 程序从 main 函数开始执行，下面的语句会按顺序运行。
	// 返回 0 表示程序正常结束。
	return 0;
}

Run result: The program ended normally, with no output in the terminal.

int main(int argc, char* argv[])
{
	// 程序从 main 函数开始执行，argc/argv 用来接收命令行参数。
	// 返回 0 表示程序正常结束。
	return 0;
}

Run result: The program ended normally, with no output in the terminal.

Among them:

• argc Indicates the number of command-line arguments, usually at least 1, because the path of the program itself is also counted as an argument.
• argv Indicates the array of command-line arguments,argv[0] is usually the path or name of the program itself,argv[1] Only then do the arguments passed in by the user begin.
• char* argv[] It can also be written as char** argv; here both express the same meaning.

Example:

#include <iostream>

using namespace std;

int main(int argc, char* argv[])
{
    // 程序从 main 函数开始执行，argc/argv 用来接收命令行参数。

	cout << "argc = " << argc << endl;

	for (int i = 0; i < argc; i++) {
		cout << "argv[" << i << "] = " << argv[i] << endl;
	}

	// 返回 0 表示程序正常结束。
	return 0;
}

Run result：see "Run result" below;argv[0] will vary with the startup method. Focus on the correspondence between argc and each argv.

Build and pass command-line arguments to run:

Linux：

cmake --build --preset linux-debug
./build/linux-debug/src/cmake_template hello 123

Windows，PowerShell：

cmake --build --preset windows-debug
./build/windows-debug/bin/cmake_template.exe hello 123

Running result：

argc = 3
argv[0] = ./build/linux-debug/src/cmake_template
argv[1] = hello
argv[2] = 123

注意：上面按 Linux 的运行路径展示结果。Windows 中程序路径通常包含 windows-debug/bin/cmake_template.exe，这属于正常的平台差异。argv[0] 的具体内容和启动方式有关，可能是 ./build/linux-debug/src/cmake_template，也可能是完整路径。实际处理参数时，通常从 argv[1] 开始读取用户输入的内容。

1.4.Comment

作用：在代码中加一些说明和解释，方便自己或其他程序员程序员阅读代码

Two formats

1. Single-line comment：// 描述信息
   • Usually placed above a line of code, or at the end of a statement, ==to explain that line of code==
2. Multi-line comment： /* 描述信息 */
   • Usually placed above a block of code, ==to provide an overall explanation of that block of code==

Tip: When compiling code, the compiler ignores the content of comments.

1.5.Variable

作用：给一段指定的内存空间起名，方便操作这段内存

Syntax:数据类型 变量名 = 初始值;

Example:

#include<iostream>
using namespace std;

int main() {

	//变量的定义
	//语法：数据类型  变量名 = 初始值

	int a = 10;

	cout << "a = " << a << endl;
	

	return 0;
}

Operation result:

a = 10

Note: C++ When creating a variable, you must give the variable an initial value, otherwise an error will occur.

1.6.Constant

作用：用于记录程序中不可更改的数据

C++Two ways to define constants

1. \#define Macro constant: #define 常量名 常量值

   • ==Usually defined at the top of the file==, representing a constant

2. const-modified variable const 数据类型 常量名 = 常量值

   • ==Usually, the keyword const is added before the variable definition== to modify the variable as a constant that cannot be changed

Example:

//1、宏常量
#define day 7

int main() {

	cout << "一周里总共有 " << day << " 天" << endl;
	//day = 8;  //报错，宏常量不可以修改

	//2、const修饰变量
	const int month = 12;
	cout << "一年里总共有 " << month << " 个月份" << endl;
	//month = 24; //报错，常量是不可以修改的
	
	

	return 0;
}

Operation result:

一周里总共有 7 天
一年里总共有 12 个月份

1.7.Keyword

**Function:** Keywords are words (identifiers) reserved in advance in C++

• When defining variables or constants, do not use keywords.

C++The keywords are as follows:

提示：在给变量或者常量起名称时候，不要用C++得关键字，否则会产生歧义。

1.8.Identifier naming rules

Function：C++When naming identifiers (variables, constants), there is a set of rules of its own.

• Identifiers cannot be keywords
• Identifiers can only consist of letters, digits, and underscores
• The first character must be a letter or an underscore
• Letters in identifiers are case-sensitive

Recommendation: When naming identifiers, strive to achieve the effect of knowing the meaning from the name, making it easier for yourself and others to read.

2.The Relationship Between C and C++ and the Learning Path

2.1.What are C and C++?

C and C++ are both general-purpose compiled programming languages. In addition, common programming languages include Python, Rust, Go, C# (pronounced C Sharp), Java, and JavaScript, among others; Shell, on the other hand, is mainly used for writing command-line and system administration scripts.

C++ It was originally developed from the C language, so the two share many similarities in basic syntax, operators, flow control, functions, and pointers. In some technical materials, toolchains, and job descriptions that involve both, people often use "C/C++" as a combined term.

However, C and C++ are now two languages with separately formulated standards and separate development, and cannot be simply understood as two versions of the same language:

1. C++ is not "fully compatible with C". Most simple C code can be migrated to C++ relatively easily, but there is still some valid C code that cannot be compiled directly as C++.
2. C++ is not just "C with classes". Modern C++ also includes templates, RAII, exceptions, lambdas, smart pointers, concurrency support, and a rich standard library.
3. C is also not an outdated language that has been replaced by C++. C is still heavily used in operating systems, embedded development, drivers, low-level libraries, and cross-language interfaces.

Therefore, C and C++ share a common foundation, but each also has its own suitable programming style. Learning one can indeed help in understanding the other, but when writing C++, one should not merely copy C-style code.

The application scenarios in the table are not rigid boundaries. For example, embedded systems can also use C++, and large applications may also include C modules. The final choice of which language to use depends on project requirements, runtime environment, existing ecosystem, and team experience.

2.2.Should I learn C first, or directly learn C++?

Learning C++ does not require completing the study of C first. For readers of this tutorial, it is recommended to directly follow the C++ learning path:

1. 先掌握变量、分支、循环、函数、数组和基本输入输出。
2. 再学习引用、指针、结构体、类和对象。
3. 接着学习 std::string、std::vector、迭代器和常用算法。
4. 然后理解 RAII、智能指针、模板、Lambda 和异常处理。
5. 最后结合 CMake、调试器、单元测试和第三方库完成真实工程。

If there is a need in the future to carry out embedded, operating system, driver, or pure C project development, then systematically supplementing the C language's arrays and pointers, memory layout, structures, preprocessor, and manual resource management will be more targeted.

This tutorial focuses on C++. When encountering syntax shared with the C language, it will be explained along the way, but examples will > as much as possible adopt a style suitable for modern C++, rather than treating C++ as a simple extension of the C language.

2.3.Supplementary Learning Materials

This tutorial mainly focuses on organizing knowledge points, engineering environments, and problems easily encountered in practice. Beginners can also learn by combining the following videos or documents:

1. C++ video tutorial
2. Peng Ge C language video
3. Runoob: C language tutorial
4. Runoob: C++ tutorial

Different materials may adopt different C++ standards and code styles. When learning the syntax, you may refer to multiple sources, but in the project of this tutorial, the C++ notation and CMake configuration given in the current chapter shall prevail.