Compilation principle of C++ virtual functions: how code is converted into machine instructions

Compilation process: preprocessing: remove comments, macro definitions, etc. Lexical analysis: Break down code into basic units (identifiers, keywords). Syntax analysis: Build a syntax tree. Semantic analysis: Check semantic correctness. Intermediate code generation: Generate platform-independent intermediate code. Code generation: Generate assembly language or machine code for the target platform. Virtual function compilation: Virtual function table generation: Creates a data structure containing a pointer to the virtual function implementation, with the address stored in the vptr of the base class object. Virtual function call: loads the vptr and uses the offset to find the correct virtual function implementation.

Compilation principle of C virtual functions: from code to machine instructions

Introduction
Virtual functions are C object-oriented programming A powerful feature that allows derived classes to override methods in base classes. This article will delve into the compilation principle of virtual functions and explain how to convert code into machine instructions.

Compilation process
The basic steps for the compiler to convert C source code into machine instructions are as follows:

1. Preprocessing: Remove comments, macro definitions and other preprocessing commands.
2. Lexical analysis: Decompose the preprocessed code into basic units called lexical units (identifiers, keywords, symbols, etc.).
3. Syntax analysis: Build a syntax tree based on lexical units to represent the structure of the code.
4. Semantic analysis: Check the semantic correctness of the syntax tree, such as type checking, symbol parsing, etc.
5. Intermediate code generation: Convert the syntax tree into a platform-independent intermediate code representation.
6. Code generation: Convert intermediate code into assembly language or machine code specific to the target platform.

Compilation of virtual functions
For virtual functions, the compiler will perform additional steps to process the virtual function table (Virtual Function Table, VFT) and virtual function calls:

1. Virtual function table generation: The compiler creates a VFT, which is a data structure that contains pointers to virtual function implementations. The address of the VFT is stored in the vptr (virtual pointer) member of the base class object.
2. Virtual function calls: When a virtual function is called, the compiler generates code to load the vptr and then uses the offset in the vptr to find and call the correct virtual function implementation.

Practical case
The following is a simple C code example showing the compilation of virtual functions:

class Base {
public:
    virtual void print() { cout << "Base::print()" << endl; }
};

class Derived : public Base {
public:
    virtual void print() override { cout << "Derived::print()" << endl; }
};

int main() {
    Base* base = new Derived();
    base->print(); // 调用派生类的 print()
    return 0;
}

The machine code generated by compilation
Compiling the above code will generate x86-64 assembly code similar to the following:

; vptr 的初始化
derived_vptr:
    .quad derived_print
    .quad base_delete

; base_print 函数
base_print:
    ; vptr 加载到寄存器
    movq (%rdx), %rcx
    ; 偏移量加载到寄存器
    movq 0x0(%rcx), %rax
    ; 执行虚函数实现
    callq *%rax

; derived_print 函数
derived_print:
    ; 打印派生类的消息
    leaq .LC0(%rip), %rdi
    call printf

.LC0:
    .string "Derived::print()"

This assembly code shows how virtual function calls are implemented by loading vptr and using offsets.

Conclusion
By deeply understanding the compilation principle of virtual functions, we can better understand how the virtual function mechanism works in object-oriented programming, and make full use of them to build flexible and powerful code.

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