Vector push_back overwrites all entries

Question

I want to use the push_back function to fill my vector with lines from a text file. But it overwrites all entries with the last line. Here is the source code:

  int main() {
    std::vector<char*> lines;
    FILE* file;
    file = fopen("textfile.txt", "r");
    const size_t max_line_length = 1000;
    char line[max_line_length + 1];
    while ( !feof(file)) {
      fgets(line, max_line_length, file);
      lines.push_back(line);
    }
    fclose(file);
 }

Hope somebody can help.

@milleniumbug He'd probably make the same mistake with them. `feof()` isn't predictive, so you have to test _after_ each input, not before. — James Kanze, Jul 18 '12 at 17:21

score 8 · Accepted Answer · answered Jul 18 '12 at 16:59

8

You are overwriting Line, and that's actually the only thing your are storing, since you never make deep copies. Try this instead:

int main() {
    std::vector<std::string> lines; // <- change this!
    FILE* file;
    file = fopen("textfile.txt", "r");
    const size_t max_line_length = 1000;
    char line[max_line_length + 1];
    while ( !feof(file)) {
      fgets(line, max_line_length, file);
      lines.push_back(line);
    }
    fclose(file);
 }

answered Jul 18 '12 at 16:59

ltjax

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3

You'll still get an extra line in the array (probably, anyway---it's not really specified). `feof` is very much like `std::basic_ios<>::eof()`; it's only use is _after_ an input has failed. The loop should be `while ( fgets( line, max_line_length, file ) )` – James Kanze Jul 18 '12 at 17:24

score 1 · Answer 2 · answered Jul 18 '12 at 17:01

You are pushing the same buffer (line) into every position of the vector. You'll need to either:

reallocate a new char* for every line using new
use strdup to copy the line buffer on every line
use std::string as @milleniumbug suggested (my favorite)

Vector push_back overwrites all entries

2 Answers2