Xcode 9+ operator new

lib.cc

#include <string>

#include <cstdio>

void* operator new(size_t s)
{
        auto p = std::malloc(s);
        std::printf("lib alloc %p - %d\n", p, int(s));
        return p;
}

void operator delete(void* p) noexcept
{
        std::printf("lib free %p\n", p);
        std::free(p);
}

void do_stuff(const char* x)
{
        std::string temp(x);
        std::printf("STR %s\n", temp.c_str());
}

main.cc

#include <string>

void do_stuff(const char*);

#if defined(MAIN_NEW)
void* operator new(size_t s)
{
        auto p = std::malloc(s);
        std::printf("main alloc %p - %d\n", p, int(s));
        return p;
}

void operator delete(void* p) noexcept
{
        std::printf("main free %p\n", p);
        std::free(p);
}
#endif



int main()
{
        std::string x(42, 'x');
        do_stuff(x.c_str());
        return 0;
}

Makefile

all: clean a.out test
liblib.dylib: lib.cc unexported.txt
        clang++ ${CXXFLAGS} -std=c++14 -dynamiclib -single_module lib.cc -o liblib.dylib -Wall  -Wl,-unexported_symbols_list,unexported.txt
a.out: main.cc liblib.dylib
         clang++ ${CXXFLAGS} -std=c++14 main.cc -llib -L .
clean:
        rm -f a.out liblib.dylib
test:
        ./a.out

Run

make
make CXXFLAGS=-O3
make CXXFLAGS=-DMAIN_NEW
make CXXFLAGS=-DMAIN_NEW\ -O3