【CPP】Introduction

0-课程特点

1. 基础知识：以点带面

2. 重点：指针和内存管理

3. 特色：

• 程序效率，提速几十倍
• 介绍OpenCV采用C++特性设计cv::Mat类
• 介绍ARM开发

1-第一个例子

Hello.cpp

//C++ example in C++11
#include 
#include 
#include 

using namespace std;

int main()
{
    vector<string> msg {"Hello", "C++", "World", "!"};
    
    for (const string& word : msg)
    {
        cout << word << " ";
    }
    cout << endl;
    return 0;
}

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Compile and run the program

• Compile hello.cpp

g++ hello.cpp
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• Initialization of msg is a C++11 extension. We need

g++ hello.cpp --std=c++11
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• Executable file can be generates as a.out. Change the output filename by -o option

g++ hello.cpp --std=c++11 -o hello
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• Execute

./hello
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2-Different Programming Languages

Assembly Languages

• List item

High Level Languages

• C: 1973
• C++: 1979
• Java:1995
• Python: 1990
• Scratch:2002

Advantages of C/C++

Development languages of most fundamental systems:

• linux
• MySQL
• OpenCV

High Effiiency

• Widely optimized compilers
• Access memory directly
• Excellent on computing

3-Complie and Link

Example

#include 

using namespace std;

int mul(int a, int b)
{
    return a * b;
}
int main()
{
    int a = 0, b = 0;
    int result = 0;

    cout << "Pick two integers:";
    cin >> a;
    cin >> b;

    result = mul(a, b);

    cout << "The result is " << result << endl;
    return 0;
}
 
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Function prototypes and definitions

• function prototypes are put into head files (*.hpp)

int mul(int a, int b);
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• function definitions normally are in source files (*.c, *.cpp)

int mul(int a, int b)
{
  return a * b;
}
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Separate the source codes into multiples files

# main.cpp
#include 
#include "mul.hpp"

using namespace std;
int main()
{
    int a = 0, b = 0;
    int result = 0;

    cout << "Pick two integers:";
    cin >> a;
    cin >> b;

    result = mul(a, b);

    cout << "The result is " << result << endl;
    return 0;
}

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# mul.cpp
#include "mul.hpp"

int mul(int a, int b)
{
    int c = a / b;
    return c * b * b;
}

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# mul.hpp
#pragma once

int mul(int a, int b);

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Compile and link

g++ -c main.cpp. // -c 只编译不链接
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g++ main.o mul.o -o mul  // 两个文件并起来
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Compilation errors

• Normally caused by grammar
• Please check the source code

Link errors

• Symbol not found
• Function mul() is misspelled to Mul()

Runtime errors

• The source code can be successfully complied and linked
• The floating point exception (divided by 0) will kill the program.
• It is a typical runtime error.

Preprocessor and Macros

Preprocessor

• The preprocessor is executed before the compilation.
• Preprocessing directives begin with a # character
• Each directive occupies one line
• preprocessing instruction
  (define, undef, include, if, ifdef, ifndef, else, elif, endif, line, error, pragma)

#include 
#define PI
3.1415926535

#if defined(_OPENMP)
#include 
#endif

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include directive

Macros

PI不是一个变量，直接替换

Simple input and output

C++ Style Output

• What is cout?

std::ostream cout;
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• cout is an object of data type stream in namespace std

cout << "hello." << endl;
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• << is an operator which is defined as follows

• end, an output-only I/O manipulator. It will output a new line character and flushes

int a;
float b;
cin >> a;
cin >> b;
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• Similarly, cin is an object of type std::instream
• “>>” is an operator

C Style Output

int v = 100;
printf("Hello, value = %d\n", v);
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• int printf(const char * format, …); is a function
• format: a string specifying how to interpret the data
• %d will let the function interpret v as an integer

C Style Input

int v;
int ret = scanf("%d", &v);
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• scanf reads data from stdin, and interpret the input as an integer and store it into v

Why the examples have no GUI?

• The programs I used all have GUI. Why the examples have no GUI?
• GUI (graphical user interface) is not mandatory
• GUI is for human begins to interact with computers
• No all programs interact with human beings
• We can also interact with the program in a command line window
• We can call a GUI library to create a graphic window by many programming languages. Surely C/C++ can create a GUI window

Command line arguments

g++ hello.cpp -o hello
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• g++ is an executable program/line
• There are three command line arguments

int main()
{
    /* ... */
}

int main(int argc, char *argv[])
{
}

int main(int argc, char **argv)
{
}
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//argc: argument count
//argv[]: 具体的参数列表

# argument.cpp
#include 

using namespace std;
int main(int argc, char * argv[])
{
    for (int i = 0; i < argc; i++)
        cout << i << ": " << argv[i] << endl;
}

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g++ argument.cpp
./a.out hello.cpp -o hello
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./a.out
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But

• I don’t like to compile a program in a command window
• IDE: Integrated development evvironment
  Visual Studio
  Apple Xcode
  Eclipes
  Clion
• Visual Srudio Code(VSCode) is an integrated development environment made by Microsoft for Windows, Linux and macOS