This is a quick guide to write a C++ std::map to a file then read the contents back again. It is useful for saving settings eg a key value / pair to a config file then reading the back again.

Writing the file is easier than reading since no parsing is involved. To make this happen all we have to do is open a file then for each item just fprintf to the file then close it again.

int WriteFile(std::string fname, std::map<std::string, std::string> *m) {
        int count = 0;
        if (m->empty())
                return 0;

        FILE *fp = fopen(fname.c_str(), "w");
        if (!fp)
                return -errno;

        for(std::map<std::string, std::string>::iterator it = m->begin(); it != m->end(); it++) {
                fprintf(fp, "%s=%s\n", it->first.c_str(), it->second.c_str());
                count++;
        }

        fclose(fp);
        return count;
}

Reading the file is a little more complex. All that needs to happen here is to open the file read from it. then for each line parse the key / pair out of the text file and add them back to the map. A quick explanation of the string parsing is below. I also find getline is useful for functions such as these so that you don't need to know the length of the line in the text file before reading it.

int ReadFile(std::string fname, std::map<std::string, std::string> *m) {
        int count = 0;
        if (access(fname.c_str(), R_OK) < 0)
                return -errno;

        FILE *fp = fopen(fname.c_str(), "r");
        if (!fp)
                return -errno;

        m->clear();

        char *buf = 0;
        size_t buflen = 0;

        while(getline(&buf, &buflen, fp) > 0) {
                char *nl = strchr(buf, '\n');
                if (nl == NULL)
                        continue;
                *nl = 0;

                char *sep = strchr(buf, '=');
                if (sep == NULL)
                        continue;
                *sep = 0;
                sep++;

                std::string s1 = buf;
                std::string s2 = sep;

                (*m)[s1] = s2;

                count++;
        }

        if (buf)
                free(buf);

        fclose(fp);
        return count;
}

The first part of the string parsing is simply to remove the newline's from the end of the file. Since this is the end of the string we can simply shorten the string by marking the newline as a NULL.

char *nl = strchr(buf, '\n');
if (nl == NULL)
 continue;
*nl = 0;

The second part that looks a little more complex is to split the line of text we have read into two strings on the '=' char. Since we have a pointer to the start of the string when we find the '=' sign and change it to NULL we turn the first
string into everything before the '='. Then when we increment the pointer to where the '=' was we have out 2nd pointer (sep) now pointing at the value section or the part of the original data after the equals.

char *sep = strchr(buf, '=');
if (sep == NULL)
        continue;
*sep = 0;
sep++;

Then all we do is convert the string's back into C++ / std::string and add them to our map and move into the next line.



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Last Modified: 12 December 2016

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