Writing a string to file in binary mode

Question

So I'm using this code to write the file(just testing now, I'm going to write a level editor later):

 
int main()
{
    ofstream file("level.bin", ios::binary);
    int ents = 1; //number of entites
    file.write((char*)&ents, sizeof(int));
    float x = 300; //x and y coords
    float y = 500;
    file.write((char*)&x, sizeof(float));
    file.write((char*)&y, sizeof(float));
    int imglength = 12; //strings are prefixed by a length
    file.write((char*)&imglength, sizeof(int));
    string img = "platform.png"; //string
    file.write(img.c_str(), sizeof(img.c_str()));
    cout << "wrote\n";
    return 0;
}

The code I'm using to load it is this:

 

void SceneManager::LoadScene(std::string filename)
{
    std::ifstream file(filename.c_str(), std::ios::binary);
    int ents;
    file.read((char*)&ents, sizeof(int));
    std::cout << ents << std::endl;
    for(int i = 0; i < ents; i++)
    {
        //read x and y coords
        float x;
        float y;
        file.read((char*)&x, sizeof(float));
        file.read((char*)&y, sizeof(float));
        std::cout << x << " " << y << std::endl;
        int imglength;
        file.read((char*)&imglength, sizeof(int));
        std::cout << imglength << std::endl;
        std::stringstream ss;
        for(int k = 0; k <= imglength; k++)
        {
            //read string
            char c;
            file.read((char*)&c, sizeof(char));
            ss << c;
        }
        std::string image = ss.str();
        std::cout << image << std::endl;
        phys_static ent;
        Def edef;
        edef.SetVal("x", x);
        edef.SetVal("y", y);
        edef.SetString("image", image);
        ent.init(edef);
        AddEntity(ent);
    }
    file.close();
}

Everything works fine except the string loading. I expect I'm writing it wrong, as instead of the platform.png it shows plattttttttt and errors out when I load the image. I'm also prefixing the string with it's length. What is the correct way to write a string to a binary file? What is the corre

Karel Petranek · Accepted Answer · 2011-03-26 21:24:35Z

10

The error is at this line:

file.write(img.c_str(), sizeof(img.c_str()));

What you want is:

file.write(img.c_str(), img.size());

The sizeof(img.c_str()) returns 4 because sizeof(char *) (the return type of c_str()) is 4 on your platform. This means first 4 characters are written and then you get just some garbage.

edited Mar 26, 2011 at 21:24

answered Mar 26, 2011 at 14:09

Karel Petranek

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6 Comments

Chris Over a year ago

That worked, except it added an extra g on the end of the string for some odd reason. I'll try decreasing the size to 11 and see if that does anything.

Konrad Rudolph Over a year ago

Null chars should not be a problem for the OP’s code (with your correction). It never assumes that '\0' acts as a string terminator and it never uses C style strings.

Karel Petranek Over a year ago

@Chris: Ah yes, that's because there should be < imglength in the for-loop reading the string - you are counting from zero.

Karel Petranek Over a year ago

@Konrad: It does assume that in the c_str() part. If img contained some '\0' characters, c_str() might return a shorter string (depending on the implementation of c_str()). But generally it should be safe because all the implementations I've seen just return the internal char buffer.

Konrad Rudolph Over a year ago

@dark It doesn’t. The c_str just creates a pointer to the characters (&img[0] would also have worked). The number of bytes to write is solely determined by the second parameter to write – whether there’d a null char in the first argument is irrelevant. Also, c_str must not return a shorter string. It must return a pointer to a char array that contains the whole string (see §21.3.6.1), plus null termination.

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