Thread: New to C++, confused about creating instance of class

Kind of not understanding how classes work. Particularly when an instance of the class is created.

I am new to C++ but I have programming in C for a few years.

Based on the book that I am learning c++ from in a constructor anything past the : is a parameter list.

I2SMEMSSampler is a derived class of I2SSampler. I pass 3 params to I2SMEMSSampler but the parameter list only accepts two parameters (in cpp file)

Can someone tell me what is going on when an object of class I2SMEMSSampler is created from main.cpp ?

Here is the code in question.

In main.cpp I have this line:

Code:

  I2SSampler *input = new I2SMEMSSampler(I2S_NUM_0, i2s_mic_pins, i2s_mic_Config);

which is declared in an h file like this:

Code:

class I2SMEMSSampler : public I2SSampler
{
private:
    i2s_pin_config_t m_i2sPins;
    bool m_fixSPH0645;

protected:
    void configureI2S();

public:
    I2SMEMSSampler(
        i2s_port_t i2s_port,
        i2s_pin_config_t &i2s_pins,
        i2s_config_t i2s_config,
        bool fixSPH0645 = false);
    virtual int read(int16_t *samples, int count);
};

in the cpp file it is defined like this, I don't understand the significance of what comes after the colon because when i create an object of the class I am giving the constructor 3 parameters as shown above in main.cpp;

Code:

I2SMEMSSampler::I2SMEMSSampler(
    i2s_port_t i2s_port,
    i2s_pin_config_t &i2s_pins,
    i2s_config_t i2s_config,
    bool fixSPH0645) : I2SSampler(i2s_port, i2s_config) 
{
    m_i2sPins = i2s_pins;
    m_fixSPH0645 = fixSPH0645;
}

I had to dig around my book which ended up having the info that explained the order of constructors and destructors.
Now I understand your example and also the one I posted.
The book had a very basic example that showed how a base class can be initialized by passing parameters to it from a derived class.
So good on the book and bad on me.
Also found, "base class objects are instantiated before a derived class" and "the sequence of destructors is opposite to that of constructors"
So now I understand the order of constructors and destructors are standardized.


Thank you john c.

Hi there,

Whilst I have complete faith in the code posted, I was just curious if this part here:

Code:

class Derived : public Base 
{
    int c_, d_;
public:
    Derived(int a, int b, int c, int d = 0) : Base(a, b), c_(c), d_(d)
    {
        std::cout << "Derived ctor\n";
    }

....

would perhaps give a compile warning as there's a pure virtual function in the base class definition? I'm aware that when you create an instance of a child class the base class's constructor is also called (usually first if I remember correctly?). I'm just curious as how the compiler would handle this given that the base is an abstract class with a pure virtual?

Thanks

You're as brilliant as ever John. That's a great and detailed reply. Thank you so much