在 C++ 中从 uint8_t 转换为 long

Question

我正在使用 map 类型来模拟记忆，即

   map<long, uint8_t> memory

其中 long 是地址，uint8_t 是内存字节。

现在我可以毫无问题地编写（签名）long 类型（我认为！）：

void Memory::writeLong(const long address, const long value)
{
    if (address < start || address + sizeof(long) > start + memorySize) {
        cout << "Memory::readLong out of range" << endl;
        throw "Memory class range error";
    }

    uint8_t *valRep = (uint8_t *) &value;
    for (int i = 0; i < sizeof(long); i++)
    {
        contents[address + i] = *((uint8_t *)(valRep + i));
    }
}

但我无法正确处理多头符号。这是阅读代码：

long Memory::readLong(const long address)
{
    long retVal = 0;

    if (address < start || address + sizeof(long) > start + memorySize) {
        cout << "Memory::readLong out of range" << endl;
        throw "Memory class range error";
    }

    uint8_t in[8];
    for (int i = 0; i < sizeof(long); i++)
    {   
        try {

            in[i] =(uint8_t) contents.at(address + i) << (i * 8);
        }
        catch (const out_of_range& err)
        {
            contents[address] = 0;
        }
    }
    memcpy(&retVal, in, 8);
    return retVal;
}

但这在尝试读取负数时会给我带来不好的结果，例如：（以书面形式：读取）

-4:1095216660732,-8:1095216660728,-5:1095216660731,-1:1095216660735,-1224:1095216660536

虽然它似乎正确读取了正数。这似乎是 2 的补码表示的问题，但我不能完全指出出了什么问题。有人能告诉我我在这里搞砸了什么吗？

谢谢

Answer 1

这里没有严重错误。一个不必要的演员表，但其他方面看起来不错。

void Memory::writeLong(const long address, const long value)
{
    if (address < start || address + sizeof(long) > start + memorySize) {
        cout << "Memory::readLong out of range" << endl;
        throw "Memory class range error";
    }

    uint8_t *valRep = (uint8_t *) &value;
    for (int i = 0; i < sizeof(long); i++)
    {
        contents[address + i] = *((uint8_t *)(valRep + i));// don't need cast
    }
}

阅读中有几处脏话：

long Memory::readLong(const long address)
{
    long retVal = 0;

    if (address < start || address + sizeof(long) > start + memorySize) {
        cout << "Memory::readLong out of range" << endl;
        throw "Memory class range error";
    }

    uint8_t in[8]; // should be uint8_t in[sizeof(retVal)];
                   // why sizeof(retVal) and not sizeof(long)
                   // someone could cut and paste to make the different typed 
                   // versions and change only retVal. Might as well lock them
    for (int i = 0; i < sizeof(long); i++) //nag nag, signed unsigned mismatch
    {   
        try {

            in[i] =(uint8_t) contents.at(address + i) << (i * 8);
            // in[i] is 8 bits. << (i * 8) ensures all but the first byte are shifted 
            // out of byte range
        }
        catch (const out_of_range& err)
        {
            contents[address] = 0;
        }
    }
    memcpy(&retVal, in, 8); // overruns retVal if retval is 32 bits. Use 
                            // sizeof(retVal) again.
    return retVal;
}

所以这不可能工作或生成 OP 报告的结果。我的猜测是 OP 有一些几乎可以工作的东西，然后开始四处乱窜试图修复它并使事情变得更糟。

我会怎么做：

编辑：事实证明，使用并集进行类型双关是非法的。糟透了，因为我认为它比指针版本更安全、更容易阅读。指针版本不违法的唯一原因可能是转换为字节的大量合法案例。

我还将 try/catch 块移到了重组循环之外，因为一旦您读取了外部内存，恢复就很顺利了。

#include <iostream>
#include <map>

std::map<long, uint8_t> contents;
void writeLong(const size_t address, const long value)
{
/* not needed for test case
    if (address < start || address + sizeof(long) > start + memorySize) {
        cout << "Memory::readLong out of range" << endl;
        throw "Memory class range error";
    }
*/
    uint8_t *valRep = (uint8_t *) &value;
    for (size_t i = 0; i < sizeof(long); i++)
    {
        contents[address + i] = *(valRep + i);
    }
}
long readLong(const size_t address)
{
// Verbotten!
//    union
//    {
//        long val;
//        uint8_t arr[sizeof(val)];
//    }retVal; // Ahhhrrrrrr! Here be endian nightmare!

    long retVal;
    uint8_t *valRep = (uint8_t *) &retVal; 
    // uglier, but legal. Consider using the no-punning version below

/* not needed for test case
    if (address < start || address + sizeof(long) > start + memorySize) {
        cout << "Memory::readLong out of range" << endl;
        throw "Memory class range error";
    }
*/
 //   uint8_t in[sizeof(long)]; replaced by evil union abuse above
    try 
    {
        for (size_t i = 0; i < sizeof(long); i++)
        {
    // Not legal
    //        retVal.arr[i] = contents.at(address + i) ;
            *(valRep + i) = contents.at(address + i);
        }
    }
    catch (const std::out_of_range& err)
    {
        retVal = 0;
    }

//    memcpy(&retVal, in, sizeof(long)); replaced by evil union abuse above
// Not legal
//    return retVal.val;
    return retVal;
}

void test(long val)
{
    writeLong(0, val);
    std::cout << "In " << val << " out " <<  readLong(0) << std::endl;
}

int main()
{
    test(-4);
    test(-8);
    test(-5);
    test(-1);
    test(-1224);
    test(0);
    test(1);
    test(0x7fffffff);
    test(0x80000000);
}

输出：

In -4 out -4
In -8 out -8
In -5 out -5
In -1 out -1
In -1224 out -1224
In 0 out 0
In 1 out 1
In 2147483647 out 2147483647
In -2147483648 out -2147483648

我会想念我的朋友双关语联盟，但是有无双关语的版本。

void writeLong(size_t address, long value)
{
// not needed for test case
//    if (address < start || address + sizeof(long) > start + memorySize) {
//        cout << "Memory::readLong out of range" << endl;
//        throw "Memory class range error";
//    }

    for (size_t i = 0; i < sizeof(long); i++)
    { // note this only preserves the proper memory layout for little endian systems
        contents[address + i] = (uint8_t)(value &0xFF);
        value >>= 8;
    }
}
long readLong(size_t address)
{
    long retVal;

// not needed for test case
//    if (address < start || address + sizeof(long) > start + memorySize) {
//        cout << "Memory::readLong out of range" << endl;
//        throw "Memory class range error";
//    }
    try
    {
        address += sizeof(long) - 1;
        retVal = contents.at(address--);
        for (size_t i = 0; i < sizeof(long) - 1; i++)
        { // this only works if the little endian memory layout is preserved
            retVal <<= 8;
            retVal += contents.at(address--);
        }
    }
    catch (const std::out_of_range& err)
    {
        retVal = 0;
    }
    return retVal;
}

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