尝试使用uint *&作为常量单位*&失败：类型为'uint8_t *'的表达式的类型为'const uint8_t *&'的引用无效初始化

问题描述

以下代码无法为我编译（gcc 4.6.3，Ubuntu 12.04）：

  #include< inttypes .H> 
 #include< stdio.h> 
 
 static inline void adjustBuffer（const uint8_t *& buf，size_t& bufSize，size_t len）
 {
 buf + = len; 
 bufSize  -  = len; 
 
 $ b uint16_t packInt（uint8_t *& buf，size_t& bufSize，int value）
 {
 size_t valueSize = sizeof（int）; 
 * reinterpret_cast< int *>（buf）= value; 
 adjustBuffer（buf，bufSize，valueSize）; 
返回valueSize; 
 
 
 bool unpackInt（const uint8_t *& buf，size_t& bufSize，int& value）
 {
 value = * reinterpret_cast< const int * >（BUF）; 
 adjustBuffer（sizeof（int））; 
返回true; 
} 
 
 int main（）
 {
 static const size_t BufSize = 100; 
 size_t bufSize = BufSize; 
 uint8_t buf [BufSize]; 
 uint8_t * buf_ptr = buf; 
 packInt（buf_ptr，bufSize，1）; 
 bufSize = BufSize; 
 int x; 
 unpackInt（buf，bufSize，x）; 
返回0; 
} 
  

我收到以下错误：

  $ make CXXFLAGS = -  Wall -gref_to_ptr 
 g ++ -Wall -g ref_to_ptr.cpp -o ref_to_ptr 
 ref_to_ptr.cpp：In function 'uint16_t packInt（uint8_t *&，size_t&; int）'：
 ref_to_ptr.cpp：15：41：错误：类型为'const uint8_t *& 'uint8_t * {aka unsigned char *}'
 ref_to_ptr.cpp：5：20：错误：在传递参数1'void adjustBuffer（const uint8_t *）'时，{aka const unsigned char *& &;; size_t&;; size_t）'
 ref_to_ptr.cpp：在函数'bool unpackInt（const uint8_t *& ;, size_t&，int&）'中：
 ref_to_ptr.cpp：22：29：error ：类型为'const uint8_t *&'的非const引用无效初始化{aka const unsigned char *&}''从'unsigned int'类型的右值
 ref_to_ptr.cpp：5：20：error：在传递'void adjustBuffer（const uint8_t *&; size_t& ，size_t）'
 ref_to_ptr.cpp：在函数'int main（）'中：
 ref_to_ptr.cpp：35：30：错误：类型为'const uint8_t *& {aka const unsigned char *&}''从类型'uint8_t * {aka unsigned char *}'$'$ b $ ref_to_ptr.cpp：19：6：错误：在传递参数1'bool unpackInt（const uint8_t *&;; size_t&; int&）'
 make：*** [ref_to_ptr] Error 1 
  

似乎编译器在将uint8_t *（uint8_t *&）引用赋值给const uint8_t *& （其中IIRC是对const指针的引用）。首先，我不明白为什么它会尝试分配一个指向uint8_t的指针而不是对指针的引用。其次，不应该转换工作？您可以将uint8_t *转换为const uint8_t *，为什么不将转换引用转换为两种类型？

当然，添加一个adjustBuffer（），它需要一个const uint8_t *&工作，但我想了解为什么

解决方案

传递 uint8_t * 作为 const uint8_t *& 参数将允许函数用<$ c替换 unint8_t * $ c> const uint8_t * 。现在，在调用者期望可修改 uint8_t * 的地方有一个 const uint8_t * 。这不是保存，因为调用者可能会在函数返回后修改指向的数据。

问题与。

The following code fails to compile for me (gcc 4.6.3, Ubuntu 12.04):

#include <inttypes.h>
#include <stdio.h>

static inline void adjustBuffer(const uint8_t *&buf, size_t &bufSize, size_t len)
{
    buf += len;
    bufSize -= len;
}

uint16_t packInt(uint8_t *&buf, size_t &bufSize, int value)
{
    size_t valueSize = sizeof(int);
    *reinterpret_cast<int *>(buf) = value;
    adjustBuffer(buf, bufSize, valueSize);
    return valueSize;
}

bool unpackInt(const uint8_t *&buf, size_t &bufSize, int &value)
{
    value = *reinterpret_cast<const int*>(buf);
    adjustBuffer(sizeof(int));
    return true;
}

int main()
{
    static const size_t BufSize = 100;
    size_t bufSize = BufSize;
    uint8_t buf[BufSize];
    uint8_t *buf_ptr = buf;
    packInt(buf_ptr, bufSize, 1);
    bufSize = BufSize;
    int x;
    unpackInt(buf, bufSize, x);
    return 0;
}

I get the following errors:

$ make CXXFLAGS="-Wall -g" ref_to_ptr
g++ -Wall -g    ref_to_ptr.cpp   -o ref_to_ptr
ref_to_ptr.cpp: In function ‘uint16_t packInt(uint8_t*&, size_t&, int)’:
ref_to_ptr.cpp:15:41: error: invalid initialization of reference of type ‘const uint8_t*& {aka const unsigned char*&}’ from expression of type ‘uint8_t* {aka unsigned char*}’
ref_to_ptr.cpp:5:20: error: in passing argument 1 of ‘void adjustBuffer(const uint8_t*&, size_t&, size_t)’
ref_to_ptr.cpp: In function ‘bool unpackInt(const uint8_t*&, size_t&, int&)’:
ref_to_ptr.cpp:22:29: error: invalid initialization of non-const reference of type ‘const uint8_t*& {aka const unsigned char*&}’ from an rvalue of type ‘unsigned int’
ref_to_ptr.cpp:5:20: error: in passing argument 1 of ‘void adjustBuffer(const uint8_t*&, size_t&, size_t)’
ref_to_ptr.cpp: In function ‘int main()’:
ref_to_ptr.cpp:35:30: error: invalid initialization of non-const reference of type ‘const uint8_t*& {aka const unsigned char*&}’ from an rvalue of type ‘uint8_t* {aka unsigned char*}’
ref_to_ptr.cpp:19:6: error: in passing argument 1 of ‘bool unpackInt(const uint8_t*&, size_t&, int&)’
make: *** [ref_to_ptr] Error 1

It seems the compiler has trouble assigning a reference to uint8_t* (uint8_t *&) to a const uint8_t *& (which IIRC is a reference to pointer to const). First, I don't understand why it tries to assign a pointer to uint8_t and not a reference to a pointer. Second, shouldn't the conversion work? You can convert uint8_t * to const uint8_t *, why wouldn't converting references to both types work?

Of course, adding an adjustBuffer() that takes a const uint8_t *& works, but I'd like to understand why

解决方案

Passing an uint8_t * as a const uint8_t *& parameter would allow the function to replace the unint8_t * with a const uint8_t *. Now there is a const uint8_t *in the place where the caller expects a modifiable uint8_t *. This is not save, since the caller might modify the pointed-to data after the function returned.

The problem is the same as in this C++ FAQ lite section.

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