C++ converting string containing non human readable data to 200 double - c++

Question

I have a string whose length is 1600 and I know that it contains 200 double. When I print out the string I get the following :Y���Vz'#��y'#��!U�}'#�-...
I would like to convert this string to a vector containing the 200 doubles.
Here is the code I tried (blobString is a string 1600 characters long):
string first_eight = blobString.substr(0, sizeof(double)); // I get the first 8 values of the string which should represent the first double
double double_value1
memcpy(&double_value1, &first_eight, sizeof(double)); // First thing I tried
double* double_value2 = (double*)first_eight.c_str(); // Second thing I tried
cout << double_value1 << endl;
cout << double_value2 << endl;
This outputs the following:
6.95285e-310
0x7ffd9b93e320
--- Edit solution---
The second method works all I had to do was look to where double_value1 was pointing.
cout << *double_value2 << endl;

Answer 1

Here's an example that might get you closer to what you need. Bear in mind that unless the numbers in your blob are in the exact format that your particular C++ compiler expects, this isn't going to work like you expect. In my example I'm building up the buffer of doubles myself.
Let's start with our array of doubles.
double doubles[] = { 0.1, 5.0, 0.7, 8.6 };
Now I'll build an std::string that should look like your blob. Notice that I can't simply initialize a string with a (char *) that points to the base of my list of doubles, as it will stop when it hits the first zero byte!
std::string double_buf_str;
double_buf_str.append((char *)doubles, 4 * sizeof(double));
// A quick sanity check, should be 32
std::cout << "Length of double_buf_str "
<< double_buf_str.length()
<< std::endl;
Now I'll reinterpret the c_str() pointer as a (double *) and iterate through the four doubles.
for (auto i = 0; i < 4; i++) {
std::cout << ((double*)double_buf_str.c_str())[i] << std::endl;
}
Depending on your circumstances you might consider using a std::vector<uint8_t> for your blob, instead of an std::string. C++11 gives you a data() function that would be the equivalent of c_str() here. Turning your blob directly into a vector of doubles would give you something even easier to work with--but to get there you'd potentially have to get dirty with a resize followed by a memcpy directly into the internal array.
I'll give you an example for completeness. Note that this is of course not how you would normally initialize a vector of doubles...I'm imagining that my double_blob is just a pointer to a blob containing a known number of doubles in the correct format.
const int count = 200; // 200 doubles incoming
std::vector<double> double_vec;
double_vec.resize(count);
memcpy(double_vec.data(), double_blob, sizeof(double) * count);
for (double& d : double_vec) {
std::cout << d << std::endl;
}
#Mooning Duck brought up the great point that the result of c_str() is not necessarily aligned to an appropriate boundary--which is another good reason not to use std::string as a general purpose blob (or at least don't interpret the internals until they are copied somewhere that guarantees a valid alignment for the type you are interested in). The impact of trying to read a double from a non-aligned location in memory will vary depending on architecture, giving you a portability concern. In x86-based machines there will only be a performance impact AFAIK as it will read across alignment boundaries and assemble the double correctly (you can test this on a x86 machine by writing then reading back a double from successive locations in a buffer with an increasing 1-byte offset--it'll just work). In other architectures you'll get a fault.
The std::vector<double> solution will not suffer from this issue due to guarantees about the alignment of newed memory built into the standard.