[Solved] initializing char* a = new char[size] does not work

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You are not null-terminating your mString data, but it is expecting to be null-terminated when you pass it to std::cout. Without that terminator, std::cout reads into surrounding memory until it encounters a random null byte (or crashes with a read access error). That is why you are seeing std::cout output random garbage after your data.

You are also not following the Rule of 3/5/0, as you are missing a default constructor, copy and move constructors, and copy and move assignment operators.

Try this:

class Practice
{
public:
    Practice(const char* a = nullptr);
    Practice(const Practice &src);
    Practice(Practice &&src);
    ~Practice();

    Practice& operator=(Practice src);

    const char* getString() const;

private:
    char* mString;
    int mSize;
};


#include "Practice.h"
#include <utility>

Practice::Practice(const char * a)
    : mSize(0)
    , mString(nullptr)
{
    if (a)
    {
        while (a[mSize] != '\0')
        {
            ++mSize;
        }
    }

    mString = new char[mSize + 1];

    for (int i = 0; i < mSize; ++i)
    {
        mString[i] = a[i];
    }

    mString[mSize] = '\0';
}

Practice::Practice(const Practice &src)
    : mSize(src.mSize)
    , mString(nullptr)
{
    mString = new char[mSize + 1];

    for (int i = 0; i < mSize; ++i)
    {
        mString[i] = src.mString[i];
    }

    mString[mSize] = '\0';
}

Practice::Practice(Practice &&src)
    : mSize(src.mSize)
    , mString(src.mString)
{
    src.mString = nullptr;
    src.mSize = 0;
}

Practice::~Practice()
{
    delete[] mString;
}

Practice& Practice::operator=(Practice src)
{
    std::swap(mString, src.mString);
    std::swap(mSize, src.mSize);
    return *this;
}

const char* Practice::getString() const
{
    return mString;
}


#include <iostream>
#include "Practice.h" 

int main()
{
    Practice p("Hello");

    std::cout << p.getString() << std::endl;

    return 0;
}

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solved initializing char* a = new char[size] does not work