c++ - Debugging a simple function

Question

So our professor gave us the task to debug this code. We can use the compiler and also try to use the gdb debugger.

#include <cstdlib>
#include <iostream>
#include <vector>

std::vector<int> reversed(const std::vector<int>& v) {
  std::vector<int> result;
  for (std::size_t i = v.size() - 1; i >= 0; --i) result.push_back(v[i]);
  return result;
}

int main(int argc, char** argv) {
  if (argc < 1) {
    std::cerr << "Usage: " << argv[0] << " [number] [number]..." << std::endl;
    return 1;
  }
  std::cerr << "Reading in " << (argc - 1) << " numbers from stdin"
            << std::endl;
  std::vector<int> numbers;
  for (std::size_t i = 0; i < argc; ++i) {
    // atoi converts a string to a number
    int number = std::stoi(argv[i]);
    numbers.push_back(number);
  }

  std::cerr << "Reversing order of numbers" << std::endl;
  auto reverse = reversed(numbers);

  for (auto n : reverse) std::cout << n << std::endl;
}

When I use g++ -Wall debug-1.cc I get this:

debug-1.cc: In function ‘int main(int, char**)’:
debug-1.cc:19:29: warning: comparison of integer expressions of different signedness: ‘std::size_t’ {aka ‘long unsigned int’} and ‘int’ [-Wsign-compare]
   19 |   for (std::size_t i = 0; i < argc; ++i) {

I also used the Debugger a put a breakpoint to lines 19, 20 and 21:

Breakpoint 1, main (argc=32767, argv=0x7ffff7fafee0 <std::wcerr>)
    at debug-1.cc:11
11  int main(int argc, char** argv) {
(gdb) c
Continuing.
Reading in 0 numbers from stdin

Breakpoint 2, main (argc=1, argv=0x7fffffffdf08) at debug-1.cc:19
19    for (std::size_t i = 0; i < argc; ++i) {
(gdb) c
Continuing.

Breakpoint 3, main (argc=1, argv=0x7fffffffdf08) at debug-1.cc:21
21      int number = std::stoi(argv[i]);
(gdb) c
Continuing.
terminate called after throwing an instance of 'std::invalid_argument'
  what():  stoi

Program received signal SIGABRT, Aborted.
__GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
50  ../sysdeps/unix/sysv/linux/raise.c: No such file or directory.

So, I guess the error lies somewhere between lines 19 and 21, but I really can't figure out what the error is. Hope you guys can help me out.

question from:https://stackoverflow.com/questions/65919217/debugging-a-simple-function

Answer 1

terminate called after throwing an instance of 'std::invalid_argument'
 what():  stoi

The function that throws an exception is std::stoi that is used to interpret a signed integer value in a string.

In this loop:

for (std::size_t i = 0; i < argc; ++i) {
    // atoi converts a string to a number
    int number = std::stoi(argv[i]);

you're trying to convert argv[0] (the program name) into an integer. If your program name isn't a number, you're out of luck.

Start the loop at 1 to only try to convert the program arguments to integers.