[nasmfunc64.asm]

global print_message_asm

section .text

;
; x64 args passed in RDI, RSI
;
print_message_asm:
    mov rdx, rsi
    mov rsi, rdi
    mov rax, 1
    mov rdi, 1
    syscall
    ret



[prog.cpp]

extern "C" void print_message_asm(const char *message, unsigned int length);

int main()
{
    print_message_asm("hello, ASM + C++ world!\n", 24);
    print_message_asm("abracadabra\n", 12);

    return 0;
}

[build x64]
$ nasm -f elf64 nasmfunc64.asm -o nasmfunc64.o
$ g++ prog.cpp nasmfunc64.o -o prog
$ ./prog