C++中的前置自增运算符与后置自增运算符

C中的前置自增运算符与后置自增运算符语义上的区别i, 后置自增, 先返回旧值, 再自增;i, 前置自增, 先自增, 再返回旧值;程序测试#includeiostreamusingnamespacestd;intmain(){inta{1},b{a},c{a};couta aendl;coutb bendl;coutc cendl;return0;}转换为汇编语言x86-64 gcc 14.2.LC0: .string a .LC1: .string b .LC2: .string c main: push rbp mov rbp, rsp sub rsp, 16 mov DWORD PTR [rbp-4], 1 mov eax, DWORD PTR [rbp-4] lea edx, [rax1] mov DWORD PTR [rbp-4], edx mov DWORD PTR [rbp-8], eax add DWORD PTR [rbp-4], 1 mov eax, DWORD PTR [rbp-4] mov DWORD PTR [rbp-12], eax mov esi, OFFSET FLAT:.LC0 mov edi, OFFSET FLAT:std::cout call std::basic_ostreamchar, std::char_traitschar std::operatorstd::char_traitschar(std::basic_ostreamchar, std::char_traitschar, char const*) mov rdx, rax mov eax, DWORD PTR [rbp-4] mov esi, eax mov rdi, rdx call std::ostream::operator(int) mov esi, OFFSET FLAT:std::basic_ostreamchar, std::char_traitschar std::endlchar, std::char_traitschar(std::basic_ostreamchar, std::char_traitschar) mov rdi, rax call std::ostream::operator(std::ostream (*)(std::ostream)) mov esi, OFFSET FLAT:.LC1 mov edi, OFFSET FLAT:std::cout call std::basic_ostreamchar, std::char_traitschar std::operatorstd::char_traitschar(std::basic_ostreamchar, std::char_traitschar, char const*) mov rdx, rax mov eax, DWORD PTR [rbp-8] mov esi, eax mov rdi, rdx call std::ostream::operator(int) mov esi, OFFSET FLAT:std::basic_ostreamchar, std::char_traitschar std::endlchar, std::char_traitschar(std::basic_ostreamchar, std::char_traitschar) mov rdi, rax call std::ostream::operator(std::ostream (*)(std::ostream)) mov esi, OFFSET FLAT:.LC2 mov edi, OFFSET FLAT:std::cout call std::basic_ostreamchar, std::char_traitschar std::operatorstd::char_traitschar(std::basic_ostreamchar, std::char_traitschar, char const*) mov rdx, rax mov eax, DWORD PTR [rbp-12] mov esi, eax mov rdi, rdx call std::ostream::operator(int) mov esi, OFFSET FLAT:std::basic_ostreamchar, std::char_traitschar std::endlchar, std::char_traitschar(std::basic_ostreamchar, std::char_traitschar) mov rdi, rax call std::ostream::operator(std::ostream (*)(std::ostream)) mov eax, 0 leave retx86 MSVC V19.38# License: MSVC Proprietary # The use of this compiler is only permitted for internal evaluation purposes and is otherwise governed by the MSVC License Agreement. # See https://visualstudio.microsoft.com/license-terms/vs2022-ga-community/ $SG35156 DB a , 00H ORG $3 $SG35157 DB b , 00H ORG $3 $SG35158 DB c , 00H _c$ -12 ; size 4 _b$ -8 ; size 4 _a$ -4 ; size 4 _main PROC push ebp mov ebp, esp sub esp, 12 ; 0000000cH mov DWORD PTR _a$[ebp], 1 mov eax, DWORD PTR _a$[ebp] mov DWORD PTR _b$[ebp], eax mov ecx, DWORD PTR _a$[ebp] add ecx, 1 mov DWORD PTR _a$[ebp], ecx mov edx, DWORD PTR _a$[ebp] add edx, 1 mov DWORD PTR _a$[ebp], edx mov eax, DWORD PTR _a$[ebp] mov DWORD PTR _c$[ebp], eax push OFFSET std::basic_ostreamchar,std::char_traitschar std::endlchar,std::char_traitschar (std::basic_ostreamchar,std::char_traitschar ) ; std::endlchar,std::char_traitschar mov ecx, DWORD PTR _a$[ebp] push ecx push OFFSET $SG35156 mov edx, DWORD PTR __imp_std::basic_ostreamchar,std::char_traitschar std::cout push edx call std::basic_ostreamchar,std::char_traitschar std::operatorstd::char_traitschar (std::basic_ostreamchar,std::char_traitschar ,char const *) ; std::operatorstd::char_traitschar add esp, 8 mov ecx, eax call DWORD PTR __imp_std::basic_ostreamchar,std::char_traitschar std::basic_ostreamchar,std::char_traitschar ::operator(int) mov ecx, eax call DWORD PTR __imp_std::basic_ostreamchar,std::char_traitschar std::basic_ostreamchar,std::char_traitschar ::operator(std::basic_ostreamchar,std::char_traitschar (__cdecl*)(std::basic_ostreamchar,std::char_traitschar )) push OFFSET std::basic_ostreamchar,std::char_traitschar std::endlchar,std::char_traitschar (std::basic_ostreamchar,std::char_traitschar ) ; std::endlchar,std::char_traitschar mov eax, DWORD PTR _b$[ebp] push eax push OFFSET $SG35157 mov ecx, DWORD PTR __imp_std::basic_ostreamchar,std::char_traitschar std::cout push ecx call std::basic_ostreamchar,std::char_traitschar std::operatorstd::char_traitschar (std::basic_ostreamchar,std::char_traitschar ,char const *) ; std::operatorstd::char_traitschar add esp, 8 mov ecx, eax call DWORD PTR __imp_std::basic_ostreamchar,std::char_traitschar std::basic_ostreamchar,std::char_traitschar ::operator(int) mov ecx, eax call DWORD PTR __imp_std::basic_ostreamchar,std::char_traitschar std::basic_ostreamchar,std::char_traitschar ::operator(std::basic_ostreamchar,std::char_traitschar (__cdecl*)(std::basic_ostreamchar,std::char_traitschar )) push OFFSET std::basic_ostreamchar,std::char_traitschar std::endlchar,std::char_traitschar (std::basic_ostreamchar,std::char_traitschar ) ; std::endlchar,std::char_traitschar mov edx, DWORD PTR _c$[ebp] push edx push OFFSET $SG35158 mov eax, DWORD PTR __imp_std::basic_ostreamchar,std::char_traitschar std::cout push eax call std::basic_ostreamchar,std::char_traitschar std::operatorstd::char_traitschar (std::basic_ostreamchar,std::char_traitschar ,char const *) ; std::operatorstd::char_traitschar add esp, 8 mov ecx, eax call DWORD PTR __imp_std::basic_ostreamchar,std::char_traitschar std::basic_ostreamchar,std::char_traitschar ::operator(int) mov ecx, eax call DWORD PTR __imp_std::basic_ostreamchar,std::char_traitschar std::basic_ostreamchar,std::char_traitschar ::operator(std::basic_ostreamchar,std::char_traitschar (__cdecl*)(std::basic_ostreamchar,std::char_traitschar )) xor eax, eax mov esp, ebp pop ebp ret 0 _main ENDP汇编语言解读.LC0: .string a 该.LC0部分是一个标签, 它提供了一个符号名称, 并定义字符串常量;GNU汇编器使用以L开头的便器被视为本地标签的约定;main:main函数栈帧建立, 把旧的基址指针压栈保存, 准备建立新栈帧;mov rbp, rsp,rbp rsp, 当前栈顶成为本函数的基准点;sub rsp, 16, 栈向下扩展 16 字节, 为 a、b、c 三个 int 留出空间int a 1mov DWORD PTR [rbp-4], 1, 把立即数1写入rbp-4的内存位置→a 1;int b amov eax, DWORD PTR [rbp-4], 把 a 的当前值1读入寄存器 eax;lea edx, [rax1], 用 lea 快速算出 eax1 2, 结果放入 edx(这是 a 的新值);mov DWORD PTR [rbp-4], edx, 把新值 2 写回 a 的内存 → a 2;mov DWORD PTR [rbp-8], eax, 把旧值 eax(1)写入 b → b 1(注意不是 2);int c aadd DWORD PTR [rbp-4],1, 直接对内存中的 a 加 1(2→3), 先改 a 再说;mov eax, DWORD PTR [rbp-4], 把已经自增后的 a(3)读入 eax;mov DWORD PTR [rbp-12],eax, 把新值 eax(3)写入 c → c 3.几个关键概念补充说明寄存器可以把eax、edx、rbp、rsp理解为 CPU 内部的草稿纸, 速度极快, 但数量有限, 变量最终住在内存里, 计算时才临时搬到寄存器.[rbp-4]是什么意思方括号表示内存地址,rbp是栈帧基址, 减去 4 就是第一个局部变量a的位置。每个int占 4 字节, 所以b在rbp-8,c在rbp-12.lea是什么lea edx, [rax1]字面意思是load effective address(加载有效地址), 但这里 GCC 借用它做加法——因为lea比add快一点, 是编译器的小聪明.DWORD PTR是什么DWORD Double Word 4 字节, 告诉 CPU 这次读写的是 4 字节宽的数据对应 C 里的int.运行结果C 标准规定, 同一条声明语句中多个变量的初始化按声明顺序执行, 所以结果是确定的.如果写成同一个表达式里同时修改和读取同一个变量, 结果就是未定义行为.cpp// 危险未定义行为int x 1;int y x x; // x 被修改了两次结果不可预测## 参考 [Compiler Explorer](https://godbolt.org/) [[C] gcc 13.2.0 - Wandbox](https://wandbox.org/) [GDB online Debugger | Compiler - Code, Compile, Run, Debug online C, C](https://www.onlinegdb.com/) [Coliru](https://coliru.stacked-crooked.com/) [C在线编译器 - 在线编译器工具](https://www.online-compiler.com/compiler/cpp) [compile visual studio c online](https://rextester.com/l/cpp_online_compiler_visual)