What kind of mistakes they make
1 : We could disable exception, today many major compilers could do that for us(g++, clang++, visual c++).
2 : If the constructor and destructor are trivial, new and delete don't need to call the constructor and destructor.
3 : The comparison is unfair, by default, new == malloc + constructor, delete = destructor + free.
For more details, you could check stack overflow.
In theory, compiler of c++ should be able to perform same performance about new and malloc, delete and free, but could the compiler do that?
Codes
c
#include <stdio.h>
#include <stdlib.h> // pulls in declaration of malloc, free
int main()
{
char *a = (char*)malloc(1024);
for(int i = 0; i != 1024; ++i){
printf("%c", a[i]);
}
free(a);
a = NULL;
return 0;
}
c++
#include <cstdio>
int main()
{
char *a = new char[1024];
for(int i = 0; i != 1024; ++i){
printf("%c", a[i]);
}
delete []a;
return 0;
}
c assembly : clang -S -O3 -mllvm --x86-asm-syntax=intel mallocAndNew00.c
.section __TEXT,__text,regular,pure_instructions
.globl _main
.align 4, 0x90
_main: ## @main
.cfi_startproc
## BB#0: ## %entry
push RBP
Ltmp3:
.cfi_def_cfa_offset 16
Ltmp4:
.cfi_offset rbp, -16
mov RBP, RSP
Ltmp5:
.cfi_def_cfa_register rbp
push R14
push RBX
Ltmp6:
.cfi_offset rbx, -32
Ltmp7:
.cfi_offset r14, -24
mov EDI, 1024
call _malloc
mov R14, RAX
mov EBX, 1
xor EDI, EDI
jmp LBB0_1
.align 4, 0x90
LBB0_2: ## %for.body.for.body_crit_edge
## in Loop: Header=BB0_1 Depth=1
movsx EDI, BYTE PTR [R14 + RBX]
inc RBX
LBB0_1: ## %for.body
## =>This Inner Loop Header: Depth=1
call _putchar
cmp EBX, 1024
jne LBB0_2
## BB#3: ## %for.end
mov RDI, R14
call _free
xor EAX, EAX
pop RBX
pop R14
pop RBP
ret
.cfi_endproc
.subsections_via_symbols
c++ assembly : clang++ -S -O3 -std=c++11 -mllvm --x86-asm-syntax=intel mallocAndNew00.cpp
.section __TEXT,__text,regular,pure_instructions
.globl _main
.align 4, 0x90
_main: ## @main
.cfi_startproc
## BB#0: ## %entry
push RBP
Ltmp3:
.cfi_def_cfa_offset 16
Ltmp4:
.cfi_offset rbp, -16
mov RBP, RSP
Ltmp5:
.cfi_def_cfa_register rbp
push R14
push RBX
Ltmp6:
.cfi_offset rbx, -32
Ltmp7:
.cfi_offset r14, -24
mov EDI, 1024
call __Znam
mov R14, RAX
mov EBX, 1
xor EDI, EDI
jmp LBB0_1
.align 4, 0x90
LBB0_2: ## %for.body.for.body_crit_edge
## in Loop: Header=BB0_1 Depth=1
movsx EDI, BYTE PTR [R14 + RBX]
inc RBX
LBB0_1: ## %for.body
## =>This Inner Loop Header: Depth=1
call _putchar
cmp EBX, 1024
jne LBB0_2
## BB#3: ## %for.end
test R14, R14
je LBB0_5
## BB#4: ## %delete.notnull
mov RDI, R14
call __ZdaPv
LBB0_5: ## %delete.end
xor EAX, EAX
pop RBX
pop R14
pop RBP
ret
.cfi_endproc
.subsections_via_symbols
This time the codes of c++ and c have some different, c++ call the assembly of new and delete(?), c call the assembly of malloc and new. c++ check the pointer is point to nullptr or not before delete, but c do not check it anyway.If you want the same behavior, call malloc and free in c++, do make sure what are you doing before you call malloc and free but not new and delete.
codes can download from github.
