how to change sizeof(std::string)

Question

I save my table data in a file with a certain pattern. I will do it with x86 and read it with same pattern. It is correct.

But if a change compiler to x64 i can't read the data from file because std::string allocation size is changing.

in x86 build:

printf("string size: %d\n", sizeof(std::string)); // output is "string size: 28"

in x64 build:

printf("string size: %d\n", sizeof(std::string)); // output is "string size: 40"

When compile mode is x64, I can't read the correct data from file due to sizeof variable changing.

Is there any way to change default size of string? if i set it 28 in both compile mode. It is enought for me.

Notes

I am not using sizeof as length i use it for verify data struct

typedef struct _USER_TABLE
{
    uint32_t ID;        
    string name;
    uint16_t age;
}



    DWORD dwNum;
    size_t i, j, iDataTypeCount = 0;
    ReadFile(hFile, &iDataTypeCount, 4, &dwNum, NULL);

    std::vector<int> offsets;
    if (iDataTypeCount > 0)
    {
        m_DataTypes.insert(m_DataTypes.begin(), iDataTypeCount, DT_NONE);
        ReadFile(hFile, &(m_DataTypes[0]), sizeof(DATA_TYPE) * iDataTypeCount, &dwNum, NULL);

        if (FALSE == MakeOffsetTable(offsets))
        {
            __ASSERT(0, "can't make offset table");
            return FALSE;
        }
        size_t iSize = offsets[iDataTypeCount];

        if (sizeof(Type) != iSize || DT_DWORD != m_DataTypes[0])
        {
            m_DataTypes.clear();
            printf("Data Type is mismatch or size is incorrect\n");
            return FALSE;
        }
    }

I use sizeof(_USER_TABLE) to verify and sizeof the sturct is changing depends on compile mode. Because string's size changing

Answer 1

There is no way to change the size of the std::string class. Nor would changing the size of std::string or even knowing that size be useful in storing data in a file because std::string is not trivially copyable.

You need to do proper serialisation. One simple approach is to store the content of buffer that std::string manages, and to rely on null terminator to determine the length of the written data.

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P.S. _USER_TABLE, __ASSERT: These identifiers are reserved to the language implementation. You should use another name for the class, and the assertion assuming you've defined it yourself.