Finding specific primitives within a binary file - c++

Question

Is there any ways to find a specific primitive within a binary file(such as fread in MATLAB or BinaryReadLists in Mathematica)? Specifically, I want to scan my file until it reaches, say a int8_t precision number, then store it in a variable, then scan for another primitive(unsigned char, double, etc..)?
I am rewriting code from MATLAB that does this, so the format of the file is known.
I want to read n bytes of only the specified type (32-bit int, char, ..) in a file. Ex: Read only the first 12 bytes of my file if they return to be 8-bit integers

Answer 1

Maybe the solution to your problem is in understanding the difference between these two doc pages:
http://www.mathworks.com/help/matlab/ref/fread.html
http://www.cplusplus.com/reference/cstdio/fread/
Both versions of fread let you pull in an array of items from a binary file. I'm assuming from your question that you know the size and shape of the array you need.
#include <stdio.h>
int main() {
const size_t NumElements = 128; // hopefully you know
int8_t myElements[NumElements];
FILE *fp = fopen("mydata.bin", "rb");
assert(fp != NULL);
size_t countRead = fread(myElements, sizeof(int8_t), NumElements, fp);
assert(countRead = NumElements);
// do something with myElements
}

Answer 2

Your question makes no sense to me, but here's a bunch of random information on how to read a binary file:
struct myobject { //so you have your data
char weight;
double value;
};
//for primitives in a binary format you simply read it in
std::istream& operator>>(std::istream& in, myobject& data) {
return in >> data.weight >> data.value;
//we don't really care about failures here
}
//if you don't know the length, that's harder
std::istream& operator>>(std::istream& in, std::vector<myobject>& data) {
int size;
in >> size; //read the length
data.clear();
for(int i=0; i<size; ++i) { //then read that many myobject instances
myobject obj;
if (in >> obj)
data.push_back(obj);
else //if the stream fails, stop
break;
}
return in;
}
int main() {
std::ifstream myfile("input.txt", std::ios_base::binary); //open a file
std::vector<myobject> array;
if (myfile >> array) //read the data!
//well that was easy
else
std::cerr << "error reading from file";
return 0;
};
Also, you can use the .seek(position) member of ifstream to skip directly to a specific point in the file, if you happen to know where to find the data you're looking for.
Oh, you just want to read the first 12 bytes of the file as 8 bit integers, and then the next 12 bytes as int32_t?
int main() {
std::ifstream myfile("input.txt", std::ios_base::binary); //open a file
std::vector<int8_t> data1(12); //array of 12 int8_t
for(int i=0; i<12; ++i) //for each int
myfile >> data1[i]; //read it in
if (!myfile) return 1; //make sure the read succeeded
std::vector<int32_t> data2(3); //array of 3 int32_t
for(int i=0; i<3; ++i) //for each int
myfile >> data2[i]; //read it in
if (!myfile) return 1; //make sure the read succeeded
//processing
}