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track_memory_allocations_overriding_new_operator.md

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Track Memory Allocations

If we override the new operator globally, we are asking the compiler to do not use the "new" operator that come with std library but use ours.

void * operator new (size_t size)
{
    std::cout<<"Allocating "<<size<<" bytes" <<std::endl;
    return malloc(size);
}

void  operator delete(void * memory, size_t size)
{
    std::cout<<"Freeing "<<size<<" bytes" <<std::endl;
    free(memory);
}

void  operator delete(void * memory )
{
    std::cout<<"Freeing " <<std::endl;
    free(memory);
}

Now if create instance of S:

std::cout<<"size of struct S is: "<<sizeof(S)<<" bytes" <<std::endl;
S* my_S=new S;
delete my_S;

The output is:

size of struct S is: 16
Allocating 16 bytes
Freeing 16 bytes

if we check the memory allocation for STL container such as strings:

std::string str;
std::cout<<"allocating and freeing memory for strings" <<std::endl;
for(std::size_t i=0;i<25;i++)
{
	str =str+std::to_string(i);
	std::cout<<"size of string is: "<<str.size()<<" bytes and string is: "<< str <<std::endl;
}

we will observer the followings in the output:

allocating and freeing memory for strings
size of string is: 1 bytes and string is: 0
size of string is: 2 bytes and string is: 01
size of string is: 3 bytes and string is: 012
size of string is: 4 bytes and string is: 0123
size of string is: 5 bytes and string is: 01234
size of string is: 6 bytes and string is: 012345
size of string is: 7 bytes and string is: 0123456
size of string is: 8 bytes and string is: 01234567
size of string is: 9 bytes and string is: 012345678
size of string is: 10 bytes and string is: 0123456789
size of string is: 12 bytes and string is: 012345678910
size of string is: 14 bytes and string is: 01234567891011
Allocating 31 bytes
size of string is: 16 bytes and string is: 0123456789101112
Allocating 31 bytes
Freeing 
size of string is: 18 bytes and string is: 012345678910111213
Allocating 31 bytes
Freeing 
size of string is: 20 bytes and string is: 01234567891011121314
Allocating 31 bytes
Freeing 
size of string is: 22 bytes and string is: 0123456789101112131415
Allocating 31 bytes
Freeing 
size of string is: 24 bytes and string is: 012345678910111213141516
Allocating 31 bytes
Freeing 
size of string is: 26 bytes and string is: 01234567891011121314151617
Allocating 31 bytes
Freeing 
size of string is: 28 bytes and string is: 0123456789101112131415161718
Allocating 31 bytes
Freeing 
size of string is: 30 bytes and string is: 012345678910111213141516171819
Allocating 33 bytes
Freeing 
size of string is: 32 bytes and string is: 01234567891011121314151617181920
Allocating 35 bytes
Freeing 
size of string is: 34 bytes and string is: 0123456789101112131415161718192021
Allocating 37 bytes
Freeing 
size of string is: 36 bytes and string is: 012345678910111213141516171819202122
Allocating 39 bytes
Freeing 
size of string is: 38 bytes and string is: 01234567891011121314151617181920212223
Allocating 41 bytes
Freeing 
size of string is: 40 bytes and string is: 0123456789101112131415161718192021222324
Freeing

This shows us that small size STL containers (m_size <= 16), would be set on stack instead of heap and after the size get bigger they would be allocated on heap, this is called The Small String Optimization

Refs: 1