What kind of mistakes they make?
1 : We do not have to take the cost of constructor and destructor, it is not a must.In other words, if your structure or class satisfy the requirements of trivial constructor and trivial destructor, you program will not invoke any constructor nor destructor.
A class or struct is trivial constructor if
- It is implicitly defined
- A has no virtual functions and no virtual base classes
- All the direct base classes of A have trivial constructors
- The classes of all the nonstatic data members of A have trivial constructors
If any of the above are false, then the constructor is nontrivial.
A class or struct is trivial destructor if
- It is implicitly defined
- All the direct base classes of A have trivial destructors
- The classes of all the nonstatic data members of A have trivial destructors
If any of the above are false, then the destructor is nontrivial.
All of the POD have trivial destructor and trivial constructor.
2 : You think constructor and destructor cost you something, but in truth they rarely do(compare with equivalent C behavior)
In theory, we don't have to take the cost of constructor and destructor, moreover, they rarely cost us anything. Theory is good, let us fallback to reality, can compiler get the job done?If you don't sure about it, measure, rather than guessing about performance.
Following codes are compiled by clang3.3 and clang++3.3 on mac OS X 10.8.5(mac mini, intel cpu).
High level codes and associative assembly of case 1
c codes
struct trivialStruct
{
int a;
float b;
double c;
};
int main()
{
struct trivialStruct A;
return 0;
}
assembly generated by command "clang -S -O2 -mllvm --x86-asm-syntax=intel trivialConstructDestruct00.c"
.section __TEXT,__text,regular,pure_instructions .globl _main .align 4, 0x90 _main: ## @main .cfi_startproc ## BB#0: ## %entry push RBP Ltmp2: .cfi_def_cfa_offset 16 Ltmp3: .cfi_offset rbp, -16 mov RBP, RSP Ltmp4: .cfi_def_cfa_register rbp lea RDI, QWORD PTR [RIP + L_.str] mov AL, 2 call _printf xor EAX, EAX pop RBP ret .cfi_endproc .section __TEXT,__cstring,cstring_literals L_.str: ## @.str .asciz "%d, %f, %f" .subsections_via_symbols
c++ codes
struct trivialStruct
{
int a;
float b;
double c;
};
int main()
{
trivialStruct A;
return 0;
}
assembly generated by command "clang++ -S -O2 -mllvm --x86-asm-syntax=intel trivialConstructDestruct00.cpp"
.section __TEXT,__text,regular,pure_instructions .globl _main .align 4, 0x90 _main: ## @main .cfi_startproc ## BB#0: ## %entry push RBP Ltmp2: .cfi_def_cfa_offset 16 Ltmp3: .cfi_offset rbp, -16 mov RBP, RSP Ltmp4: .cfi_def_cfa_register rbp lea RDI, QWORD PTR [RIP + L_.str] mov AL, 2 call _printf xor EAX, EAX pop RBP ret .cfi_endproc .section __TEXT,__cstring,cstring_literals L_.str: ## @.str .asciz "%d, %f, %f" .subsections_via_symbols
Apparently, c++
High level codes and associative assembly of case 2
c
#include#include struct trivialStruct { int *a; float *b; float *c; }; void construct_trivial_struct(struct trivialStruct *data) { data->a = (int*)malloc(sizeof(int)); data->b = (float*)malloc(sizeof(float)); data->c = (float*)malloc(sizeof(float)); *data->a = 100; *data->b = 200; *data->c = 300; } void destruct_trivial_struct(struct trivialStruct *data) { free(data->a); free(data->b); free(data->c); data->a = NULL; data->b = NULL; data->c = NULL; } int main() { struct trivialStruct A; construct_trivial_struct(&A); printf("%d, %f, %f", *A.a, *A.b, *A.c); destruct_trivial_struct(&A); return 0; }
assembly generated by command "clang -S -O2 -mllvm --x86-asm-syntax=intel trivialConstructDestruct00.c"
.section __TEXT,__text,regular,pure_instructions .globl _construct_trivial_struct .align 4, 0x90 _construct_trivial_struct: ## @construct_trivial_struct .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 R15 push R14 push RBX push RAX Ltmp6: .cfi_offset rbx, -40 Ltmp7: .cfi_offset r14, -32 Ltmp8: .cfi_offset r15, -24 mov RBX, RDI mov EDI, 4 call _malloc mov R14, RAX mov QWORD PTR [RBX], R14 mov EDI, 4 call _malloc mov R15, RAX mov QWORD PTR [RBX + 8], R15 mov EDI, 4 call _malloc mov QWORD PTR [RBX + 16], RAX mov DWORD PTR [R14], 100 mov DWORD PTR [R15], 1128792064 mov DWORD PTR [RAX], 1133903872 add RSP, 8 pop RBX pop R14 pop R15 pop RBP ret .cfi_endproc .globl _destruct_trivial_struct .align 4, 0x90 _destruct_trivial_struct: ## @destruct_trivial_struct .cfi_startproc ## BB#0: ## %entry push RBP Ltmp12: .cfi_def_cfa_offset 16 Ltmp13: .cfi_offset rbp, -16 mov RBP, RSP Ltmp14: .cfi_def_cfa_register rbp push RBX push RAX Ltmp15: .cfi_offset rbx, -24 mov RBX, RDI mov RDI, QWORD PTR [RBX] call _free mov RDI, QWORD PTR [RBX + 8] call _free mov RDI, QWORD PTR [RBX + 16] call _free mov QWORD PTR [RBX + 16], 0 mov QWORD PTR [RBX + 8], 0 mov QWORD PTR [RBX], 0 add RSP, 8 pop RBX pop RBP ret .cfi_endproc .section __TEXT,__literal8,8byte_literals .align 3 LCPI2_0: .quad 4641240890982006784 ## double 200 LCPI2_1: .quad 4643985272004935680 ## double 300 .section __TEXT,__text,regular,pure_instructions .globl _main .align 4, 0x90 _main: ## @main .cfi_startproc ## BB#0: ## %entry push RBP Ltmp18: .cfi_def_cfa_offset 16 Ltmp19: .cfi_offset rbp, -16 mov RBP, RSP Ltmp20: .cfi_def_cfa_register rbp lea RDI, QWORD PTR [RIP + L_.str] movsd XMM0, QWORD PTR [RIP + LCPI2_0] movsd XMM1, QWORD PTR [RIP + LCPI2_1] mov ESI, 100 mov AL, 2 call _printf xor EAX, EAX pop RBP ret .cfi_endproc .section __TEXT,__cstring,cstring_literals L_.str: ## @.str .asciz "%d, %f, %f" .subsections_via_symbolsc++
#include#include struct trivialStruct { trivialStruct(); ~trivialStruct(); int *a; float *b; float *c; }; trivialStruct::trivialStruct() : a((int*)malloc(sizeof(int))), b((float*)malloc(sizeof(float))), c((float*)malloc(sizeof(float))) { *a = 100; *b = 200; *c = 300; } trivialStruct::~trivialStruct() { free(a); free(b); free(c); a = nullptr; b = nullptr; c = nullptr; } int main() { trivialStruct A; printf("%d, %f, %f", *A.a, *A.b, *A.c); return 0; }
.section __TEXT,__text,regular,pure_instructions .globl __ZN13trivialStructC1Ev .align 4, 0x90 __ZN13trivialStructC1Ev: ## @_ZN13trivialStructC1Ev .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 R15 push R14 push RBX push RAX Ltmp6: .cfi_offset rbx, -40 Ltmp7: .cfi_offset r14, -32 Ltmp8: .cfi_offset r15, -24 mov RBX, RDI mov EDI, 4 call _malloc mov R14, RAX mov QWORD PTR [RBX], R14 mov EDI, 4 call _malloc mov R15, RAX mov QWORD PTR [RBX + 8], R15 mov EDI, 4 call _malloc mov QWORD PTR [RBX + 16], RAX mov DWORD PTR [R14], 100 mov DWORD PTR [R15], 1128792064 mov DWORD PTR [RAX], 1133903872 add RSP, 8 pop RBX pop R14 pop R15 pop RBP ret .cfi_endproc .globl __ZN13trivialStructC2Ev .align 4, 0x90 __ZN13trivialStructC2Ev: ## @_ZN13trivialStructC2Ev .cfi_startproc ## BB#0: ## %entry push RBP Ltmp12: .cfi_def_cfa_offset 16 Ltmp13: .cfi_offset rbp, -16 mov RBP, RSP Ltmp14: .cfi_def_cfa_register rbp push R15 push R14 push RBX push RAX Ltmp15: .cfi_offset rbx, -40 Ltmp16: .cfi_offset r14, -32 Ltmp17: .cfi_offset r15, -24 mov RBX, RDI mov EDI, 4 call _malloc mov R14, RAX mov QWORD PTR [RBX], R14 mov EDI, 4 call _malloc mov R15, RAX mov QWORD PTR [RBX + 8], R15 mov EDI, 4 call _malloc mov QWORD PTR [RBX + 16], RAX mov DWORD PTR [R14], 100 mov DWORD PTR [R15], 1128792064 mov DWORD PTR [RAX], 1133903872 add RSP, 8 pop RBX pop R14 pop R15 pop RBP ret .cfi_endproc .globl __ZN13trivialStructD1Ev .align 4, 0x90 __ZN13trivialStructD1Ev: ## @_ZN13trivialStructD1Ev .cfi_startproc ## BB#0: ## %entry push RBP Ltmp21: .cfi_def_cfa_offset 16 Ltmp22: .cfi_offset rbp, -16 mov RBP, RSP Ltmp23: .cfi_def_cfa_register rbp push RBX push RAX Ltmp24: .cfi_offset rbx, -24 mov RBX, RDI mov RDI, QWORD PTR [RBX] call _free mov RDI, QWORD PTR [RBX + 8] call _free mov RDI, QWORD PTR [RBX + 16] call _free mov QWORD PTR [RBX + 16], 0 mov QWORD PTR [RBX + 8], 0 mov QWORD PTR [RBX], 0 add RSP, 8 pop RBX pop RBP ret .cfi_endproc .globl __ZN13trivialStructD2Ev .align 4, 0x90 __ZN13trivialStructD2Ev: ## @_ZN13trivialStructD2Ev .cfi_startproc ## BB#0: ## %entry push RBP Ltmp28: .cfi_def_cfa_offset 16 Ltmp29: .cfi_offset rbp, -16 mov RBP, RSP Ltmp30: .cfi_def_cfa_register rbp push RBX push RAX Ltmp31: .cfi_offset rbx, -24 mov RBX, RDI mov RDI, QWORD PTR [RBX] call _free mov RDI, QWORD PTR [RBX + 8] call _free mov RDI, QWORD PTR [RBX + 16] call _free mov QWORD PTR [RBX + 16], 0 mov QWORD PTR [RBX + 8], 0 mov QWORD PTR [RBX], 0 add RSP, 8 pop RBX pop RBP ret .cfi_endproc .section __TEXT,__literal8,8byte_literals .align 3 LCPI4_0: .quad 4641240890982006784 ## double 200 LCPI4_1: .quad 4643985272004935680 ## double 300 .section __TEXT,__text,regular,pure_instructions .globl _main .align 4, 0x90 _main: ## @main .cfi_startproc ## BB#0: ## %entry push RBP Ltmp34: .cfi_def_cfa_offset 16 Ltmp35: .cfi_offset rbp, -16 mov RBP, RSP Ltmp36: .cfi_def_cfa_register rbp lea RDI, QWORD PTR [RIP + L_.str] movsd XMM0, QWORD PTR [RIP + LCPI4_0] movsd XMM1, QWORD PTR [RIP + LCPI4_1] mov ESI, 100 mov AL, 2 call _printf xor EAX, EAX pop RBP ret .cfi_endproc .section __TEXT,__cstring,cstring_literals L_.str: ## @.str .asciz "%d, %f, %f" .subsections_via_symbolsWe could see that the part of construct and destruct are same as the assembly generated by c.Although the compiler generate two pieces of constructor and destructor for us, this do not mean the codes will become fatter or slower, because the linker could remove the duplicate the codes for us, even if they are not removed(hard to believe this will happen in modern linker), they will take some space in memory, but will never be evaluated.
One of the way to verify this is separate the declaration and definition of the struct, generate the assembly by same command and look at the codes, you will find that the assembly only call for one symbol of the constructor. The final step is verify the size of the exe, if the linker haven't removed duplicate codes, the size of the exe should be fatter. The other solution is download a disassemble to analyze the exe by yourself.
In the conclusion, most of the times we have to construct or destruct the object, so constructor and destructor rarely cost us anything(compare with equivalent c behavior).Even you really don't them, you could disable them by your wish, nothing stop you from gaining full control in c++.
Codes are available on github.
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