c++Basic knowledge

This chapter is mainly for people who do not have base of C++, mainly to explain the common C++ grammar and common classes in our system :

Class

Mention C++, you have to mention the class first. Don't think about it too complicated. Just think of it as a structure in the C language.

For example :

// C
struct Position {
    int left;
    int top;
    int width;
    int height;
};

// c++
class Position {
public:
    int left;
    int top;
    int width;
    int height;
};

Define variables :

// C
struct Position pos;

// c++
Position pos;

Operating variables :

// C is the same as c++
pos.left = 0;

The class has more concepts of inheritance, polymorphism, overloading and access permissions than the structure in the C language. For people who are only familiar with the C language, they don’t need to pay too much attention to these details, as long as they master how to use it;

In addition, functions can be directly defined in class. The structure in C language is to define function pointers. This is a bit different. After class defines the function, it can be used like operating variables. Here Give an example commonly used in our framework :

// Set the text content, where mTextView1Ptr is a pointer variable of type ZKTextView
mTextView1Ptr->setText("Hello");

Commonly used class

string class

The string class actually encapsulates the string and provides a lot of functions. Person who are only familiar with the C language only need to know one function: c_str(), this function will return the characters in the string class string data, here is also an example commonly used in our framework:

// Input box callback function
static void onEditTextChanged_Edittext1(const std::string &text) {
    // The return value type of c_str() function : const char *
    const char *pStr = text.c_str();

    // Then you can operate like ordinary strings, such as getting the string length strlen(pStr), etc.

}

Give another example of obtaining the content of a text control :

// std is the namespace, std::string means to use the string class under std, don’t worry too much 
// When encountering the string class, we can refer to the following definition
std::string text = mTextView1Ptr->getText();

// The subsequent operations are the same
const char *pStr = text.c_str();

formatted output function "snprintf"

Function prototype :

  int snprintf(char* dest_str,size_t size,const char* format,...);

Function :

The variable parameters (...) are formatted into a string according to format, and then copied to dest_str. (1) If the length of the formatted string is less than size, copy the string to dest_str, and add a string terminator ('\0') after it;
(2) If the length of the formatted string >= size, only (size-1) characters are copied to dest_str, and a string terminator ('\0') is added after it, and the return value is the desired the length of the written string.

Required header files :

  #include <stdio.h>

Formatting parameters

• Specifier
  %d Decimal signed integer
  %u Decimal unsigned integer
  %f Floating point
  %s String
  %c Character
  %p Pointer value
  %e Exponential floating point
  %x, %X Unsigned integer in hexadecimal
  %o Unsigned integer in octal
  %g Output the value according to the smaller output length in %e or %f type
  %p Output address
  %lu Output long integer %llu 64-bit unsigned integer

• Description
  (1).You can insert a number between "%" and the letter to indicate the maximum field width.
  For example: %3d means to output a 3-digit integer, which is not enough to be right-justified.
  %9.2f represents a floating-point number with a field width of 9, where the decimal place is 2 and the integer place is 6,

  the decimal point occupies one digit, which is not enough for 9 digits to be right-justified.
  %8s means output a string of 8 characters, which is not enough to right-justify 8 characters.
  If the length of the string or the number of integers exceeds the specified field width, it will be output according to its actual length.
  But for floating-point numbers, if the number of integers exceeds the specified width of integers, it will be output as actual integers;
  If the number of decimal places exceeds the specified width of decimal places, the output will be rounded according to the specified width.
  In addition, if you want to add some zeros before the output value, you should add a zero before the field width term.
  For example: %04d means that when outputting a value less than 4 digits, 0 will be added to the front to make the total width 4 digits.
  If a floating point number is used to represent the output format of characters or integers, the number after the decimal point represents the maximum width, and the number before the decimal point represents the minimum width.
  For example: %6.9s means to display a string with a length not less than 6 and not greater than 9. If it is greater than 9, the content after the 9th character will be deleted.
  (2).You can add a lowercase letter l between "%" and the letter to indicate that the output is a long number.
  For example: %ld means output long integer
  %lf means output double floating point number
  (3) You can control the output to be left-aligned or right-aligned, that is, add a "-" sign between "%" and the letter, indicating that the output is left-aligned, otherwise it is right-aligned.
  For example: %-7d means that the output 7-bit integer is left-justified
  %-10s means output 10 characters left-justified

• Special specifier
  \n Wrap
  \f Clear screen and change page
  \r Carriage return
  \t Tab character

Example

• Direct output integer

  char buf[64] = {0};
  snprintf(buf, sizeof(buf), "%d", 314);
  LOGD("%s", buf);//Log output buf
  

  The log output is

  314
  

• Control the number of digits of the integer

  char buf[64] = {0};
  snprintf(buf, sizeof(buf), "%05d", 314); //Format as 5 digits, less than 5 digits, add 0 in front
  LOGD("%s", buf);//Log output buf string
  

  The log output is

  00314
  

• Output floating point number directly

  char buf[64] = {0};
  snprintf(buf, sizeof(buf), "%f", 3.14); 
  LOGD("%s", buf);//Log output buf string
  

  The log output is

  3.140000
  

• Control output floating point number format

  char buf[64] = {0};
  //Output decimals, a total of 6 characters wide (including the decimal point), 3 decimal places, two integer digits, and 0 if   //the integer is less than two digits
  snprintf(buf, sizeof(buf), "%06.3f", 3.14);
  LOGD("%s", buf);//Log output buf string
  

  The log output is

  03.140