原始指针 [raw pointers]

指针是一个变量，用于存储对象的内存地址。
指针广泛应用于 C 和 C++：

• 在堆上分配新对象
• 通过参数将某些函数传递给其他函数
• 迭代/遍历数组或其他数据结构的元素

    int* p = nullptr; // declare pointer and initialize it
                      // so that it doesn't store a random address
    int i = 5;
    p = &i; // assign pointer to address of object
    int j = *p; // dereference p to retrieve the value at its address

    MyClass* mc = new MyClass(); // allocate object on the heap
    mc->print(); // access class member
    delete mc; // delete object (please don't forget!)

    // declare a C-style string. Compiler adds terminating '\0'.
    const char* str = "Hello world";

    const int c = 1;
    const int* pconst = &c; // declare a non-const pointer to const int
    const int c2 = 2;
    pconst = &c2;  // OK pconst itself isn't const
    const int* const pconst2 = &c;
    // pconst2 = &c2; // Error! pconst2 is const.

当定义函数时，尽量将指针参数标记为 const，除非你期望函数能够修改传入的对象。一般来讲，const 引用更适合用来传递对象给函数，除非对象可能为 nullptr。

函数指针使函数能够被传递给其他函数，在 C 风格代码中可用于回调。现代 C++ 使用 lambda 表达式来达到同样地目的。

C 风格代码示例：

#include <iostream>
#include <string>

class MyClass
{
public:
    int num;
    std::string name;
    void print() { std::cout << name << ":" << num << std::endl; }
};

// Accepts a MyClass pointer
void func_A(MyClass* mc)
{
    // Modify the object that mc points to.
    // All copies of the pointer will point to
    // the same modified object.
    mc->num = 3;
}

// Accepts a MyClass object
void func_B(MyClass mc)
{
    // mc here is a regular object, not a pointer.
    // Use the "." operator to access members.
    // This statement modifies only the local copy of mc.
    mc.num = 21;
    std::cout << "Local copy of mc:";
    mc.print(); // "Erika, 21"
}


int main()
{
    // Use the * operator to declare a pointer type
    // Use new to allocate and initialize memory
    MyClass* pmc = new MyClass{ 108, "Nick" };

    // Prints the memory address. Usually not what you want.
    std:: cout << pmc << std::endl;

    // Copy the pointed-to object by dereferencing the pointer
    // to access the contents of the memory location.
    // mc is a separate object, allocated here on the stack
    MyClass mc = *pmc;

    // Declare a pointer that points to mc using the addressof operator
    MyClass* pcopy = &mc;

    // Use the -> operator to access the object's public members
    pmc->print(); // "Nick, 108"

    // Copy the pointer. Now pmc and pmc2 point to same object!
    MyClass* pmc2 = pmc;

    // Use copied pointer to modify the original object
    pmc2->name = "Erika";
    pmc->print(); // "Erika, 108"
    pmc2->print(); // "Erika, 108"

    // Pass the pointer to a function.
    func_A(pmc);
    pmc->print(); // "Erika, 3"
    pmc2->print(); // "Erika, 3"

    // Dereference the pointer and pass a copy
    // of the pointed-to object to a function
    func_B(*pmc);
    pmc->print(); // "Erika, 3" (original not modified by function)

    delete(pmc); // don't forget to give memory back to operating system!
   // delete(pmc2); //crash! memory location was already deleted
}

指针算法和数组

指针和数组是紧密联系的，当一个数组以值传递给函数时，传递的是指向第一个元素的指针，下面的示例演示了指针和数组的重要属性：

• sizeof 操作符返回数组以字节为单位的大小。
• 要确定元素的个数，可以用元素的大小值去除数组的总大小。
• 当数组传递给函数时，将退化为指针类型。
• sizeof 操作符用在指针上时返回时指针的大小，在 x86 系统上是4字节，在 x64 系统上是8字节。

#include <iostream>

void func(int arr[], int length)
{
    // returns pointer size. not useful here.
    size_t test = sizeof(arr);

    for(int i = 0; i < length; ++i)
    {
        std::cout << arr[i] << " ";
    }
}

int main()
{

    int i[5]{ 1,2,3,4,5 };
    // sizeof(i) = total bytes
    int j = sizeof(i) / sizeof(i[0]);
    func(i,j);
}

非 const 指针可以进行算术操作以指向不同的内存地址: ++, +=, -= and --。

这对于数组尤其是无类型的缓冲区特别有用:

• void* 以一个 char (1 byte)的大小递增。
• 有类型的指针以类型的大小递增。

以下示例演示了指针如何通过算数操作 Windows 位图的像素点。
注意 new 和 delete 的使用，以及解引用操作。

#include <Windows.h>
#include <fstream>

using namespace std;

int main()
{

    BITMAPINFOHEADER header;
    header.biHeight = 100; // Multiple of 4 for simplicity.
    header.biWidth = 100;
    header.biBitCount = 24;
    header.biPlanes = 1;
    header.biCompression = BI_RGB;
    header.biSize = sizeof(BITMAPINFOHEADER);

    constexpr int bufferSize = 30000;
    unsigned char* buffer = new unsigned char[bufferSize];

    BITMAPFILEHEADER bf;
    bf.bfType = 0x4D42;
    bf.bfSize = header.biSize + 14 + bufferSize;
    bf.bfReserved1 = 0;
    bf.bfReserved2 = 0;
    bf.bfOffBits = sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER); //54

    // Create a gray square with a 2-pixel wide outline.
    unsigned char* begin = &buffer[0];
    unsigned char* end = &buffer[0] + bufferSize;
    unsigned char* p = begin;
    constexpr int pixelWidth = 3;
    constexpr int borderWidth = 2;

    while (p < end)
    {
            // Is top or bottom edge?
        if ((p < begin + header.biWidth * pixelWidth * borderWidth)
            || (p > end - header.biWidth * pixelWidth * borderWidth)
            // Is left or right edge?
            || (p - begin) % (header.biWidth * pixelWidth) < (borderWidth * pixelWidth)
            || (p - begin) % (header.biWidth * pixelWidth) > ((header.biWidth - borderWidth) * pixelWidth))
        {
            *p = 0x0; // Black
        }
        else
        {
            *p = 0xC3; // Gray
        }
        p++; // Increment one byte sizeof(unsigned char).
    }

    ofstream wf(R"(box.bmp)", ios::out | ios::binary);

    wf.write(reinterpret_cast<char*>(&bf), sizeof(bf));
    wf.write(reinterpret_cast<char*>(&header), sizeof(header));
    wf.write(reinterpret_cast<char*>(begin), bufferSize);

    delete[] buffer; // Return memory to the OS.
    wf.close();
}

void* 指针

void 指针指向原始内存地址。有时需要使用 void* 指针，例如在 C++ 代码和 C 函数之间传递时。

当有类型的指针转换为 void 指针时，保存的内存地址是不变的。然而，类型信息会丢失，所以你不能进行加减操作。内存地址是可以转型(cast)的，例如，从 MyClass* 到 void* 然后回到 MyClass*。

这样的操作本身就是容易出错的，必须很谨慎。现代 C++ 几乎在所有场景中都不建议使用 void 指针。

//func.c
void func(void* data, int length)
{
    char* c = (char*)(data);

    // fill in the buffer with data
    for (int i = 0; i < length; ++i)
    {
        *c = 0x41;
        ++c;
    }
}

// main.cpp
#include <iostream>

extern "C"
{
    void func(void* data, int length);
}

class MyClass
{
public:
    int num;
    std::string name;
    void print() { std::cout << name << ":" << num << std::endl; }
};

int main()
{
    MyClass* mc = new MyClass{10, "Marian"};
    void* p = static_cast<void*>(mc);
    MyClass* mc2 = static_cast<MyClass*>(p);
    std::cout << mc2->name << std::endl; // "Marian"

    // use operator new to allocate untyped memory block
    void* pvoid = operator new(1000);
    char* pchar = static_cast<char*>(pvoid);
    for(char* c = pchar; c < pchar + 1000; ++c)
    {
        *c = 0x00;
    }
    func(pvoid, 1000);
    char ch = static_cast<char*>(pvoid)[0];
    std::cout << ch << std::endl; // 'A'
    operator delete(p);
}

函数指针

C 风格代码中，函数指针主要用来将一个函数传递给另一个函数。这允许调用方可以定制函数的行为而不去修改他。

现代 C++ 中，lambda 表达式提供了同样地功能并保证了更高的安全性和其他优点。

函数指针声明规定了指向的函数必须要有如下签名：

// Declare pointer to any function that...

// ...accepts a string and returns a string
string (*g)(string a);

// has no return value and no parameters
void (*x)();

// ...returns an int and takes three parameters
// of the specified types
int (*i)(int i, string s, double d);

示例：

#include <iostream>
#include <string>

using namespace std;

string base {"hello world"};

string append(string s)
{
    return base.append(" ").append(s);
}

string prepend(string s)
{
    return s.append(" ").append(base);
}

string combine(string s, string(*g)(string a))
{
    return (*g)(s);
}

int main()
{
    cout << combine("from MSVC", append) << "\n";
    cout << combine("Good morning and", prepend) << "\n";
}