Debugging (Language-Specific)

C++

Assertions & Warnings

GCC Compilation Options

You can also check what options Errichto and ecnerwala use.

Warning Options

Such as the following:

-Wall -Wextra -Wshadow -Wconversion -Wfloat-equal -Wduplicated-cond -Wlogical-op

You can check the CF blog above for descriptions of these.

-Wextra

Some extra warning flags that are not enabled by -Wall

Doesn't seem so useful in a contest setting? Except maybe the following:

-Wunused-parameter
-Wunused-but-set-parameter

-Wshadow

Avoid variable shadowing!

Other Options

Let's give some examples of what each of these do.

-fsanitize=undefined

The following prog.cpp gives a segmentation fault.

1#include <bits/stdc++.h>
2using namespace std;
3
4int main() {
5    vector<int> v;
6    cout << v[-1];
7}

g++ prog.cpp -o prog -fsanitize=undefined && ./prog produces:

/usr/local/Cellar/gcc/9.2.0_1/include/c++/9.2.0/bits/stl_vector.h:1043:34: runtime error: pointer index expression with base 0x000000000000 overflowed to 0xfffffffffffffffc
zsh: segmentation fault  ./prog

Another example with prog.cpp as the following:

1#include <bits/stdc++.h>
2using namespace std;
3
4int main() {
5    int v[5];
6    cout << v[5];
7}

g++ prog.cpp -o prog -fsanitize=undefined && ./prog produces:

prog.cpp:6:13: runtime error: index 5 out of bounds for type 'int [5]'
prog.cpp:6:13: runtime error: load of address 0x7ffee0a77a94 with insufficient space for an object of type 'int'
0x7ffee0a77a94: note: pointer points here
  b0 7a a7 e0 fe 7f 00 00  25 b0 a5 0f 01 00 00 00  b0 7a a7 e0 fe 7f 00 00  c9 8c 20 72 ff 7f 00 00
              ^

-fsanitize=undefined also catches integer overflow. Let prog.cpp be the following:

1#include <bits/stdc++.h>
2using namespace std;
3
4int main() {
5    int x = 1<<30;
6    cout << x+x;
7}

g++ prog.cpp -o prog -fsanitize=undefined && ./prog produces:

prog.cpp:6:15: runtime error: signed integer overflow: 1073741824 * 2 cannot be represented in type 'int'

We can also use -fsanitize=undefined with -fsanitize-recover. Error recovery for -fsanitize=undefined is turned on by default, but

The -fno-sanitize-recover= option can be used to alter this behavior: only the first detected error is reported and program then exits with a non-zero exit code.

So if prog.cpp is as follows:

1#include <bits/stdc++.h>
2using namespace std;
3
4int main() {
5    cout << (1<<32) << endl;
6    cout << (1<<32) << endl;
7    cout << (1<<32) << endl;
8}

then

g++ -fsanitize=undefined prog.cpp -o prog && ./prog

produces:

prog.cpp: In function 'int main()':
prog.cpp:5:12: warning: left shift count >= width of type [-Wshift-count-overflow]
    5 |  cout << (1<<32) << endl;
      |           ~^~~~
prog.cpp:6:12: warning: left shift count >= width of type [-Wshift-count-overflow]
    6 |  cout << (1<<32) << endl;
      |           ~^~~~
prog.cpp:7:12: warning: left shift count >= width of type [-Wshift-count-overflow]
    7 |  cout << (1<<32) << endl;
      |           ~^~~~
prog.cpp:5:12: runtime error: shift exponent 32 is too large for 32-bit type 'int'
0
prog.cpp:6:12: runtime error: shift exponent 32 is too large for 32-bit type 'int'
0
prog.cpp:7:12: runtime error: shift exponent 32 is too large for 32-bit type 'int'
0

while

g++ -fsanitize=undefined -fno-sanitize-recover prog.cpp -o prog && ./prog

produces:

prog.cpp: In function 'int main()':
prog.cpp:5:12: warning: left shift count >= width of type [-Wshift-count-overflow]
    5 |  cout << (1<<32) << endl;
      |           ~^~~~
prog.cpp:6:12: warning: left shift count >= width of type [-Wshift-count-overflow]
    6 |  cout << (1<<32) << endl;
      |           ~^~~~
prog.cpp:7:12: warning: left shift count >= width of type [-Wshift-count-overflow]
    7 |  cout << (1<<32) << endl;
      |           ~^~~~
prog.cpp:5:12: runtime error: shift exponent 32 is too large for 32-bit type 'int'
zsh: abort      ./prog

-fsanitize=address -g

The following prog.cpp gives a segmentation fault.

1#include <bits/stdc++.h>
2using namespace std;
3
4int main() {
5    vector<int> v;
6    cout << v[-1];
7}

g++ prog.cpp -o prog -fsanitize=address && ./prog produces:

For more helpful information we should additionally compile with the -g flag, which generates a file containing debugging information based on the line numbering of the program. -fsanitize=address can then access the file at runtime and give meaningful errors. This is great because it helps diagnose (or "sanitize" if you will) errors such as out of bounds, exceptions, and segmentation faults, even indicating precise line numbers. Feel free to delete the debug file after the run of course.

Another example with prog.cpp as the following:

1#include <bits/stdc++.h>
2using namespace std;
3
4int main() {
5    int v[5];
6    cout << v[5];
7}

g++ prog.cpp -o prog -fsanitize=address -g && ./prog produces:

-D_GLIBCXX_DEBUG

The following prog.cpp gives a segmentation fault.

1#include <bits/stdc++.h>
2using namespace std;
3
4int main() {
5    vector<int> v;
6    cout << v[-1];
7}

g++ prog.cpp -o prog -D_GLIBCXX_DEBUG && ./prog produces: