访问另一个结构体指针内的结构体指针内的元素

只是想了解结构体的嵌套指针如何工作。如何访问另一个指向结构的指针中的结构指针中的元素？谢谢。

#include <iostream>

typedef struct
{
  unsigned int            a;
  unsigned int            b;
  unsigned int            c;
} inner;

typedef struct
{
  unsigned int            d;
  unsigned int            e;
  inner * in;
} outer;

/* Function to use struct */
void test(outer * out)
{
  //Update the value of out->in->a.
  out->in->a = 5;
}

int main() {
  outer * out;
  test(out);

  std::cout << "My updated value: " << out->in->a << std::endl;
}

我收到“分段错误（核心已转储）”。

编辑：根据建议进行了更改。

#include <iostream>

typedef struct
{
    unsigned int            a;
    unsigned int            b;
    unsigned int            c;
} inner;

typedef struct
{
    unsigned int            d;
    unsigned int            e;
    inner * in = new inner();
} outer;

/* Function to use struct */
void test(outer * out)
{
    //Update the value of out->d.
    out->d = 3; 
    //Update the value of out->in->a.
    out->in->a = 5;
}

int main() {

    outer out; 
    outer* p = &out; 

    test(p);

    std::cout << "My updated value for d: " << p->d << std::endl;
    std::cout << "My updated value for a: " << p->in->a << std::endl;
}

我现在得到了更新的正确值。不过我现在有一个警告

“警告：非静态数据成员初始值设定项仅适用于 -std=c++11 或 -std=gnu++11”

inner * in = new inner();

我该如何解决这个警告？

对于初学者来说，C 和 C++ 是不同的语言。

在任何情况下，您都必须在访问其元素抛出指针之前分配结构。否则，使用未初始化的指针或不指向有效结构对象的指针会导致程序的未定义行为。

这是一个演示性 C 程序，展示了如何使用指针访问结构的元素

#include <stdlib.h>
#include <stdio.h>

typedef struct
{
  unsigned int            a;
  unsigned int            b;
  unsigned int            c;
} inner;

typedef struct
{
  unsigned int            d;
  unsigned int            e;
  inner * in;
} outer;


int main( void ) 
{
    outer *out = malloc( sizeof( outer ) );

    if ( ( out != NULL ) && ( out->in = malloc( sizeof( inner ) ) ) )
    {
        out->in->a = 1;
        out->in->b = 2;
        out->in->c = 3;

        printf( "out->in = { a = %d, b = %d, c = %d }\n", out->in->a, out->in->b, out->in->c );

        free( out->in );
        free( out );
    }

    return 0;
}

它的输出是

out->in = { a = 1, b = 2, c = 3 }

在 C++ 中，程序可以是这样的

#include <iostream>

typedef struct
{
  unsigned int            a;
  unsigned int            b;
  unsigned int            c;
} inner;

typedef struct
{
  unsigned int            d;
  unsigned int            e;
  inner * in;
} outer;

int main() 
{
    outer *out = new outer;

    out->in = new inner;

    out->in->a = 1;
    out->in->b = 2;
    out->in->c = 3;

    std::cout << "out->in = { a = " << out->in->a
              << ", b = " << out->in->b
              << ", c = " << out->in->c
              << " }" << std::endl;

    delete out->in;
    delete out;

    return 0;
}

其输出将与上图所示相同。

至于这个功能

void test(outer *out)
{
  //Update the value of out->in->a.
  out->in->a = 5;
}

然后两个指针out and in必须指向类型的有效对象outer and inner.