C++ Core Guidelines: Lifetime Safety

This is a cross-post from www.ModernesCpp.com.

The lifetime safety profile in the C++ core guidelines boils down to one issue: don't dereference a possible invalid pointer. A pointer may be invalid because it is not initialised or the nullptr. A pointer may also point beyond its rage or to a deleted object.

Lifetime Safety

When you don't dereference a possible invalid pointer, the impact to your program is according to the C++ core guidelines manifold:

• eliminates one of the major sources of nasty errors in C++
• eliminates a major source of potential security violations
• improves performance by eliminating redundant “paranoia” checks
• increases confidence in correctness of code
• avoids undefined behavior by enforcing a key C++ language rule

Honestly, dealing with pointers is part of a bigger story: ownership. Ownership means that at each point in time, it must be obvious, who is responsible for managing the lifetime of an object. Roughly speaking, C++11 support six kinds of ownership:

• Local objects. The C++ runtime, as the owner automatically manages the lifetime of these resources. The same holds for global objects or members of a class. The guidelines calls them scoped objects.
• References: I'm not the owner. I only borrowed the resource that cannot be empty.
• Raw pointers: I'm not the owner. I only borrowed the resource that can be can be empty. I must not delete the resource.
• std::unique_ptr: I'm the exclusive owner of the resource. I may explicitly release the resource.
• std::shared_ptr: I share the resource with other shared pointer. I may explicitly release my shared ownership.
• std::weak_ptr: I'm not the owner of the resource, but I may become temporary the shared owner of the resource by using the method std::weak_ptr::lock.

Compare this fine-grained ownership semantic with a raw pointer. Now you know, what I like about modern C++.

Now, you may ask yourself: Having rules is fine but how can I check that my code follows these rules? Thanks to the Guidelines Support Library (GSL) the rules of the C++ core guidelines can automatically be checked. 

Checking the Rules of the Guidelines

The GSL is a small library for supporting the guidelines of the C++ core guidelines. They are already a few implementations of the GSL available.

The GSL is a header only library, hence, you can use the functions and types of the library quite easily. The best-known implementation is the one from Microsoft, hosted at Github: Microsoft/GSL. The Microsoft version requires C++14 support and runs on various platforms. Here are a few quite popular platforms:

• Windows using Visual Studio 2015
• Windows using Visual Studio 2017
• Windows using Visual Studio 2019
• Windows using Clang/LLVM 3.6
• Windows using Clang/LLVM 7.0.0
• Windows using GCC 5.1
• Windows using Intel C++ Compiler 18.0
• GNU/Linux using Clang/LLVM 3.6-3.9
• GNU/Linux using Clang/LLVM 4.0
• GNU/Linux using Clang/LLVM 5.0
• GNU/Linux using Clang/LLVM 6.0
• GNU/Linux using Clang/LLVM 7.0
• GNU/Linux using GCC 5.1

Let's see what I can achieve with the GSL. Here is a program, which breaks Type Safety, Bounds Safey, and Lifetime Safety.

Break of Type Safety, Bounds Safety, and Lifetime Safety

// gslCheck.cpp

#include <iostream>

void f(int* p, int count) {
}

void f2(int* p) {
    int x = *p;
}

int main() {

    // Break of type safety
    // use of a c-cast
    double d = 2;
    auto p = (long*)&d;
    auto q = (long long*)&d;

    // Break of bounds safety
    // array to pointer decay
    int myArray[100];
    f(myArray, 100);

    // Break of Lifetime Safety
    // a is not valid
    int* a = new int;
    delete a;
    f2(a);

}

The comments in the source code document my issues. Now, let me start Visual Studio 2019 and show my steps to visualise the issues.

Enable Code Analysis on Build

You have to enable the Checkbox. Per default, the three Type Safety, Bounds Safety, and Lifetime Safety rules are not part of the Microsoft Native Recommended Rules.

Configure your Active Rules

As you can see from the screenshot, I create my ruleset CheckProfile, which consisted of the rules C++ Core Guidelines Bounds Rules, C++ Core Guidelines Type Rules, and C++ Core Guidelines Lifetime Rules.

Run Code Analysis on Solution

Applying my set of rules on the code example was quite promising.

All issues are found. For each issue such as the first one, I get the line number (17), and the rule of the affected profile (type.4).

Suppress Warnings

Sometimes, you want to suppress specific warnings. You can achieve this with attributes. My next example applies two times an array to pointer decay. Only the second call should give a warning.

// gslCheckSuppress.cpp

#include <iostream>

void f(int* p, int count) {
}

int main() {

    int myArray[100];
    
    // Break of bounds safety
    [[gsl::suppress(bounds.3)]] {   // suppress warning
        f(myArray, 100);
    }

    f(myArray, 100);                // warning           

}

The attribute gsl::suppress(bounds.3) behaves as expected. It's only valid in its scope. The second break of bounds safety is displayed.

What's next?

I skip the next section of the C++ core guidelines because I already wrote a post to the Guidelines Support Library. I assume the next chapter will be quite controversial: naming and layout rules.