Making the Compiler Do the Work

Compilers have lots of options that affect the compilation. So far, with g++ we’ve just been using -o to name the executable. But we can also have the compiler warn us about problematic C++ code we may have written. A useful set of options is:

g++ -Wall -Wextra -Wshadow -Wpedantic

Here’s a brief summary:

• -Wall : this turns on options that warn about things most users deem problematic.

• -Wextra : enabled additional warnings that most people think are good to check for.

• -Wpedantic : makes sure that you conform to the language standard and not rely on any extensions that the compiler might support.

Here’s an example of a case of using the wrong type in a loop that can be caught with these flags:

Listing 38 types.cpp

#include <array>

int main() {

    std::array<double, 10> x;

    for (int i = 0; i < x.size(); ++i) {
        x[i] = i;
    }

}

Here’s an example of catching unused variables:

Listing 39 unused.cpp

int main() {

    int x, y, z;

    x = 1;
    y = x * 2;

}

Here’s an example of catching a fallthrough in a switch statement:

Listing 40 fallthrough.cpp

#include <iostream>
#include <cmath>

int main() {

    double x{};
    std::string sign{};

    std::cout << "enter a number: ";
    std::cin >> x;

    const auto sgn = static_cast<int>(std::copysign(1.0, x));

    switch (sgn) {

    case -1:
        sign = "negative";
    case 0:
        sign = "none";
        break;
    default:
        sign = "positive";
    }

    std::cout << "sign = " << sign << std::endl;
}