dental

C++ Core Guidelines: More Rules for Expressions

I know this post’s headline is a bit boring: More Rules for Expressions. Honestly, this post is about code hygiene because I will mainly write about pointers.

 

dental

Let’s have a look at my plan for today.

I will start with a significant rule.

ES.42: Keep use of pointers simple and straightforward

Let me cite the words of the guidelines: “Complicated pointer manipulation is a major source of errors.”. Why should we care? Of course, our legacy code is full of functionality, such as this example:

 

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    void f(int* p, int count)
    {
        if (count < 2) return;
    
        int* q = p + 1;    // BAD
    
        int n = *p++;      // BAD
    
        if (count < 6) return;
    
        p[4] = 1;          // BAD
    
        p[count - 1] = 2;  // BAD
    
        use(&p[0], 3);     // BAD
    }
    
    int myArray[100];     // (1)
    
    f(myArray, 100),      // (2)
    

     

    The main issue with this code is that the caller must provide the correct length of the C-array. If not, we have undefined behavior.

    Think about the last lines (1) and (2) for a few seconds. We start with an array and remove its type information by passing it to the function f. This process is called an array-to-pointer decay and is the reason for many errors. Maybe we had a bad day, and we count the number of elements wrong, or the size of the C-array changed. Anyway, the result is always the same: undefined behavior. The same argumentation will also hold for a C-string.

    What should we do? We should use the right data type. The Guidelines suggest using gsl::spantype from the Guidelines Support Library (GSL). Have a look here:

    void f(span<int> a) // BETTER: use span in the function declaration
    {
        if (a.length() < 2) return;
    
        int n = a[0];      // OK
    
        span<int> q = a.subspan(1); // OK
    
        if (a.length() < 6) return;
    
        a[4] = 1;          // OK
    
        a[count - 1] = 2;  // OK
    
        use(a.data(), 3);  // OK
    }
    

     

    Fine! gsl::span checks at run-time its boundaries. Additionally, the Guidelines Support Library has a free function at for accessing the elements of an gsl::span. 

    void f3(array<int, 10> a, int pos) 
    {
        at(a, pos / 2) = 1;       // OK
        at(a, pos - 1) = 2;       // OK
    }
    

     

    I know your issue. Most of you don’t use the Guidelines Support Library. No problem. It’s quite easy to rewrite the functions f and f3 using the container std::array and the method std::array::at. Here we are:

    // spanVersusArray.cpp
    
    #include <algorithm>
    #include <array>
    
    void use(int*, int){}
    
    void f(std::array<int, 100>& a){
    
        if (a.size() < 2) return;
    
        int n = a.at(0);      
    
        std::array<int, 99> q;
        std::copy(a.begin() + 1, a.end(), q.begin());      // (1)
    
        if (a.size() < 6) return;
    
        a.at(4) = 1;          
    
        a.at(a.size() - 1) = 2;
    
        use(a.data(), 3); 
    }
    
    void f3(std::array<int, 10> a, int pos){
        a.at(pos / 2) = 1;      
        a.at(pos - 1) = 2; 
    }
    
    int main(){
    
        std::array<int, 100> arr{};
    
        f(arr);
        
        std::array<int, 10> arr2{};
     
        f3(arr2, 6);
    
    }
    

     

    The std::array::at Operator will check at runtime its bounds. If pos >= size(), you will get an std::out_of_range exception. Looking carefully at the spanVersusArray.cpp program, you will notice two issues. First, the expression (1) is more verbose than the gsl::span version and second, the size of the std::array is part of the signature of the function f. This is bad. I can only use f with the type std::array<int, 100>.  In this case, the checks of the array size inside the function are superfluous. 

    To your rescue, C++ has templates; therefore, it’s easy to overcome the type restrictions but stay type-safe.

     

    // at.cpp
    
    #include <algorithm>
    #include <array>
    #include <deque>
    #include <string>
    #include <vector>
    
    template <typename T>
    void use(T*, int){}
    
    template <typename T>
    void f(T& a){
    
        if (a.size() < 2) return;
    
        int n = a.at(0);      
    
        std::array<typename T::value_type , 99> q;                 // (4)
        std::copy(a.begin() + 1, a.end(), q.begin());     
    
        if (a.size() < 6) return;
    
        a.at(4) = 1;          
    
        a.at(a.size() - 1) = 2;
    
        use(a.data(), 3);                                          // (5)
    }
    
    int main(){
    
        std::array<int, 100> arr{};                                             
        f(arr);                                                    // (1)
        
        std::array<double, 20> arr2{};
        f(arr2);                                                   // (2)
        
        std::vector<double> vec{1, 2, 3, 4, 5, 6, 7, 8, 9};
        f(vec);                                                    // (3)
        
        std::string myString= "123456789";
        f(myString);                                               // (4)
        
        // std::deque<int> deq{1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
        // f(deq);                                                 // (5)
        
    }
    

     

    Now, the function f works for std::array’s of different sizes and types (lines (1) and (2)) but also for a std::vector(3) or a std::string (4). This container has in common that its data is stored in a contiguous memory block. This will not hold std::deque; therefore, the call a.data() in expression (5) fails. A std::deque is a kind of doubly-linked list of small memory blocks.

     deque

    The expression T::value_type (5) helps me get each container’s underlying value type. T is a so-called dependent type because T is a type parameter of the function template f. This is the reason I have to give the compiler a hint that T::value_type is a type: typename T::value_type.

    ES.45: Avoid “magic constants”; use symbolic constants

    This is obvious: A symbolic constant says more than a magic constant. 

    The guidelines start with a magic constant, continue with a symbolic constant, and finish with a range-based for loop. 

    for (int m = 1; m <= 12; ++m)        // don't: magic constant 12
        cout << month[m] << '\n';
    
    
    
                      // months are indexed 1..12 (symbolic constant)
    constexpr int first_month = 1;
    constexpr int last_month = 12;
    
    for (int m = first_month; m <= last_month; ++m)        // better
        cout << month[m] << '\n';
    
    
    
    for (auto m : month)          // the best (ranged-based for loop)
        cout << m << '\n';
    

     

    In the case of the ranged-based for loop, it is not possible to make an off-by-one error.  

    Let me directly jump to the rule ES.47. I want to put the rules for conversion, including ES.46, in a separate post.

    ES.47: Use nullptr rather than 0 or NULL

    There are many reasons to use a nullptr instead of the number 0 or the macro NULL. In particular, 0 or NULL will not work in generic. I have already written a post about these three kinds of null pointers. Here are the details: The Null Pointer Constant nullptr.

    What’s next?

    How many explicit casts do we have in modern C++? Maybe your number is four, but this is the wrong number. In C++11, we have six explicit casts. When I Include the GSL, we have eight explicit casts. I will write about the eight casts in the next post.

     

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