c++ - 将默认字符串指针传递给函数

Question

我的fuu.h中有一个函数的原型(prototype)。如果有，我想传递默认值

void MyFunction(
    int iError = 0,
    std::string * MyProblemString = ""
    );

// Some attemts:

void MyFunction(
    int iError = 0,
    std::string * MyProblemString = std::string{""};    // does not work to, complains about missing '*' 
    );

void MyFunction(
    int iError = 0,
    std::string * MyProblemString = &std::string{""};   // does not work to, refering address to temporary value
    );

void MyFunction(
    int iError = 0,
    std::string * MyProblemString = *std::string{""};   // does not work to, overloaded operator not known
    );

void MyFunction(
    int iError = 0,
    std::string * MyProblemString = "";     // could not convert ‘""’ from ‘const char [1]’ to ‘std::__cxx11::string* {aka std::__cxx11::basic_string<char>*}’
    );

in my c-file is written:

void MyFunction(
    int iError,
    std::string * MyProblemString,)
{
    // do some stuff
}

有了 int，它工作正常。我怎样才能用字符串做同样的事情？

有一些其他构造的示例，但传递指针不起作用。 const reference default-value

谢谢

Answer 1

我认为您不了解指针的作用。我会尽力提供帮助。

Pointers

int number;      // An int
int * pointer;   // A valid pointer to an int
int* pointer;    // Also a valid pointer to an int
int *pointer;    // Also a valid pointer to an int

“number”是一个整数类型的命名变量，具有足够大的内存块来存储分配给它的 int。这 block 内存被赋予了一个内存地址。

指针基本上就像一个 int，除了它们存储的数字是另一个变量的内存地址 - 在您的例子中是“数字”。

& 运算符将为您提供您使用它的变量的内存地址。

&number 

这将为您提供 int“数字”的内存地址。

pointer = &number;  // Pointer now contains the memory address of "number"

现在，如果您尝试像使用 int 一样使用指针，它会为您提供“数字”的地址，而不是其内容。要访问您的指针指向的任何内容，请在其前面加上 *。

    void main()
{
    int number = 56;            

    int* pointer = number;      // INVALID: Pointer now pointing at memory location "56"
    int* pointer = &number;     // Valid: Pointer now pointing at memory location of number

    int* pointer;               // DANGEROUS, DO NOT LEAVE HANGING POINTERS -- leads to memory access violations

    int *pointer = nullptr;     // Safely initialise unused pointer
    int size = 32;
    pointer = new int;          // Allocates a block of memory the size of 1 int. Pointer now points at this memory
    pointer = new int[size];    // Allocates an array of 32 ints to pointer. Pointer now points to this block of memory
    pointer = new int[8];       // Allocates another array to pointer. Pointer now points at the new array, old array has nothing pointing at it and is lost in memory!
                                // This is a memory leak. AVOID MEMORY LEAKS AT ALL COSTS, they can produce some of the hardest-to-find bugs you'll come across

    delete pointer;             // Deletes chunk of memory the size of 1 int at the memory pointer is pointing at
    delete[] pointer;           // Deletes array of memory following the location pointer is pointing at

    // IMPORTANT: ALWAYS use delete with new, delete[] with new[]
    // IMPORTANT: NEVER use new or new[] without its delete counterpart, or you will encounter memory leaks

    // USEFUL:
    if (pointer != nullptr)
    {
        // Do stuff
    }
    // Doing this will ensure you don't act upon memory you aren't supposed to mess with


    // Print these to console to gain a better understanding.
    std::cout << number << std::endl;
    std::cout << &number << std::endl;
    std::cout << pointer << std::endl;
    std::cout << *pointer << std::endl;

    system("pause");
}

注意输出:

Output on my machine (the address will vary when you run it):

56           // number
0035F7E0     // &number
0035F7E0     // pointer
56           // *pointer

Note that "&number" and "pointer" print the same thing

您可以使用 & 和 * 将指针分配给任何东西，包括指向其他​​指针的指针，指向其他指针的指针，指向其他指针的指针，指向您想要的任何东西。像这样的事情越深入，它就会变得越困惑，但关键是(双关语意想不到的)指针是 C++ 中使用的最通用和最方便(但也很危险——内存泄漏、访问冲突、啊哈)的东西之一。

希望这对您有所帮助，如果您不明白或者这不能帮助您理解您的问题，请回复此答案。