Reading binary file into char array in c++

Question

std::streampos size;
char * memblock;

std::ifstream input ("A.JPG", std::ios::in|std::ios::binary|std::ios::ate);

if (input.is_open())
{
    size = input.tellg();
    memblock = new char [size];
    input.seekg (0, std::ios::beg);
    input.read (memblock, size);
    input.close();

    std::cout << "[INPUT]the entire file content is in memory " << sizeof(memblock) << " \n";

}
delete[] memblock;

I would like to use the ifstream to read A.JPG (28KB) and save it into the array memblock. But why do the size of the memblock is 4 instead of 28403 while the variable size is equal to 28403?

Thank you.

@πάντα ῥεῖ Can you elaborate please? I was thinking, that this case is equal to stackoverflow.com/a/37539/5247040 — Roman Zaitsev
– Roman Zaitsev, Commented Nov 26, 2015 at 10:09
@RomanZaytsev, calling sizeof(*memblock) will be similar to calling sizeof(memblock[0]) which is to say sizeof(char). — txtechhelp
– txtechhelp, Commented Nov 26, 2015 at 10:13
@RomanZaytsev sizeof() cannot be used to determine the size of an array allocated at runtime, calculation is done at compile time . Your sample applies for statically allocated arrays. — πάντα ῥεῖ
– πάντα ῥεῖ, Commented Nov 26, 2015 at 10:14

Marko Popovic · Accepted Answer · 2015-11-26 10:02:03Z

0

Because memblock is a pointer, so the sizeof operator evaluates to the size of the pointer variable, which is 4 bytes.

answered Nov 26, 2015 at 10:02

Marko Popovic

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1 Comment

user44 Over a year ago

Thanks for your answer but I do not have enough reputation to +1 your answer :(. But anyway, your answer makes me to understand the reason.

user44 · Accepted Answer · 2015-11-26 15:55:39Z

0

Thanks all and finally I used the vector instead. Because seems it is hard to display the result I want (length of the actual char array)

std::vector <char> memblock(0);
  if (input.is_open())
  {
    size = input.tellg();
    //memblock = new char [size];
    memblock.resize(size);
    input.seekg (0, std::ios::beg);
    input.read (&memblock[0], size);
    input.close();


    //std::cout << "[INPUT]the entire file content is in memory " << ((char *)(&memblock+1) - (char *)memblock) / (sizeof(memblock[0])) << " \n";
    std::cout << "[INPUT]the entire file content is in memory " << memblock.size() << " \n";

answered Nov 26, 2015 at 15:55

user44

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2 Comments

Marko Popovic Over a year ago

There is nothing wrong with this version that uses a vector but this is not the answer to your original question. You said you want to read the jpeg into an array, you did not ask for suggestions regarding a different approach. Your question was: "why do the size of the memblock is 4 instead of 28403", and you were given a correct answer. Next time when someone gives you an answer, please mark it as accepted. After all, we are using a part of our spare time to help you.

user44 Over a year ago

@Marko Popovic Sorry for that, actually I still want to know how to get the length of it. And this is not the answer maybe, and I just wanna share the alternative way to achieve the same goal. I think my question is not good, I should ask How to get the length rather than "why". But anyway, really thanks for your comment and answer. I will mark it as answer, thank you.

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