Writing and reading std::string to file

[versed canyon]

I am trying to write a std::vector<std::string> to file and read it out again.

The writing looks like this:

std::ifstream f(fname, std::ios::binary);

num = _resultColumnNames.size();
f.write(reinterpret_cast<const char*>(&num), sizeof(size_t));
for (size_t i = 0; i < num; i++) {
  std::string resultColumnName = _resultColumnNames[i];
  size_t resultColumnNameLength = resultColumnName.length();
  f.write(reinterpret_cast<const char*>(&resultColumnNameLength), sizeof(size_t));
  f.write(resultColumnName.c_str(), resultColumnNameLength);
}

1. Get the vector size to know how many std::string to write
2. Write the size to file
3. For every std::string write its length and characters to file

The reading looks like this:

f.seekg(posResultColumnNames);
for (size_t i = 0; i < numResultColumnNames; i++) {
  size_t resultColumnNameLength;
  std::string resultColumnName;
    
  f.read(reinterpret_cast<char*>(&resultColumnNameLength), sizeof(size_t));
  resultColumnName.reserve(resultColumnNameLength);
    
  f.read(reinterpret_cast<char*>(&resultColumnName[0]), resultColumnNameLength);
    
  LOG(INFO) << "[GEOMCACHE] resultColumnNameLength: " << resultColumnNameLength;
  LOG(INFO) << "[GEOMCACHE] resultColumnName: " << resultColumnName;
}

1. Seek to where the data begins
2. Read the std::string length
3. Reserve that many characters for the std::string
4. Read the characters

The length of the std::string is properly read for every std::string, but the std::string itself is always empty. I don't know if it already goes wrong when writing the data or if it is only a reading problem. Does someone know why this doesn't work? Thank you!

[worthy viper]

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[subtle gull]

You need to use ::Resize instead of ::Reserve. You cant access an index that is beyond ::Size of the string. ::Reserve doesnt change ::Size, it changes ::Capacity

#include <iostream>
#include <fstream>
#include <istream>
#include <string>
#include <vector>

int main()
{
    std::vector<std::string> _resultColumnNames = { "hi", "my", "name", "is", "noah" };
    std::ofstream f("test", std::ios::binary);

    size_t num = _resultColumnNames.size();
    f.write(reinterpret_cast<const char*>(&num), sizeof(size_t));
    for (size_t i = 0; i < num; i++) {
        std::string resultColumnName = _resultColumnNames[i];
        size_t resultColumnNameLength = resultColumnName.length();
        f.write(reinterpret_cast<const char*>(&resultColumnNameLength), sizeof(size_t));
        f.write(resultColumnName.c_str(), resultColumnNameLength);
    }

    f.close();

    std::ifstream ifs("test", std::ios::binary);
    size_t numResultColumnNames;
    ifs.read(reinterpret_cast<char*>(&numResultColumnNames), sizeof(size_t));
    for (size_t i = 0; i < numResultColumnNames; i++) {
        size_t resultColumnNameLength;
        std::string resultColumnName;

        ifs.read(reinterpret_cast<char*>(&resultColumnNameLength), sizeof(size_t));
        resultColumnName.resize(resultColumnNameLength);

        ifs.read(reinterpret_cast<char*>(&resultColumnName[0]), resultColumnNameLength);

        std::cout << "[GEOMCACHE] resultColumnNameLength: " << resultColumnNameLength << "\n";
        std::cout << "[GEOMCACHE] resultColumnName: " << resultColumnName << "\n";
    }

    return 0;
}

this works

also, you seem to be using ifstream for writing? ifstream stands for input file stream, so you can only read.

ifstream doesnt even have the ::Write function for me

so asside from the syntax problems with the streams, if you fix the reserve to resize the string should be copied correctly