Understanding behavior of overloading operator new in shared library

I am working on a shared library and application on Centos platform [clang++, llvm3.9.0 and libc++] , and both library and application overload their own operator new and operator delete.

Everything is working fine except for 1 case. On calling copy constructor of std::string always calling operator new of application side:

Here is the senario:

std::string str1 ( "A very strange issue on CentOS using clang and libc++" ); //operator new of library side called.

std::string str2(str1); //operator new of application side called. WHY??

operator delete on both cases are called for library side.

Here are the logs when running the following code:

====================================================
operator new in shared library
operator new called Application side
operator delete in shared library
operator delete in shared library
====================================================

Shared Library side operator new and delete:

    void * operator new ( size_t len ) throw ( std::bad_alloc )
    {
        printf("operator new in shared library\n");
        void * mem = malloc( len );
        if ( (mem == 0) && (len != 0) ) throw std::bad_alloc();
        return mem;
    }

    void * operator new[] ( size_t len ) throw ( std::bad_alloc )
    {
        printf("operator new[] in shared library\n");
        void * mem = malloc( len );
        if ( (mem == 0) && (len != 0) ) throw std::bad_alloc();
        return mem;
    }

    void operator delete ( void * ptr ) throw()
    {
        printf("operator delete in shared library\n");
        if ( ptr != 0 ) free ( ptr );
    }

    void operator delete[] ( void * ptr ) throw()
    {
        printf("operator delete[] in shared library\n");
        if ( ptr != 0 ) free ( ptr );
    }

Application side operator new and operator delete:

void * operator new ( size_t len ) throw ( std::bad_alloc )
{
    void * mem = malloc ( len );
    printf("operator new called Application side\n");
    if ( (mem == 0) && (len != 0) ) throw std::bad_alloc();
        return mem;
}

void * operator new[] ( size_t len ) throw ( std::bad_alloc )
{
    void * mem = malloc ( len );
    printf("operator new[] called Application side\n");
    if ( (mem == 0) && (len != 0) ) throw std::bad_alloc();
        return mem;
}

void operator delete ( void * ptr ) throw()
{
    printf("operator delete[] called Application side\n");
    if ( ptr != 0 )free ( ptr );
}

void operator delete[] ( void * ptr ) throw()
{
    printf("operator delete[] called Application side\n");
    if ( ptr != 0 ) free ( ptr );
}

Please help.