C++ Core Guidelines: Rules for Conversions and Casts

What has narrowing conversion and casts in common? They are often the source of errors; therefore, I will write about them today.

Here are the rules from the guidelines.

Narrowing conversion is a conversion of a value, including the loss of its precision. Most of the time that is not what you want.

ES.46: Avoid narrowing conversions

Here are a few examples from the guidelines.

double d = 7.9;
int i = d;    // bad: narrowing: i becomes 7
i = (int) d;  // bad: we're going to claim this is still not explicit enough

void f(int x, long y, double d)
{
    char c1 = x;   // bad: narrowing
    char c2 = y;   // bad: narrowing
    char c3 = d;   // bad: narrowing
}

If you want a narrowing conversion, you should do it explicitly, not implicitly, according to the Python rule from The Zen of Python: Explicit is better than implicit. The guideline support library (GSL) has two casts to express your intent: gsl::narrow_cast and gsl::narrow.

double d = 7.9;
i = narrow_cast<int>(d);   // OK (you asked for it): narrowing: i becomes 7
i = narrow<int>(d);        // OK: throws narrowing_error

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The gsl::narrow_cast performs the cast, and the gsl::narrow cast throws an exception if a narrowing conversion happens.

Most of the time, a narrowing conversion happened secretly. How can you protect yourself from this? Use the power of the curly braces:

// suppressNarrowingConversion.cpp

void f(int x, long y, double d){
    char c1 = {x};   
    char c2 = {y};   
    char c3 = {d};   
}

int main(){

  double d = {7.9};         
  int i = {d};    

  f(3, 3l, 3.0);
}

All initializations are put into curly braces. According to the C++11 standard, the compiler has to warn you if a narrowing conversion happens.

Explicit is better than implicit. This will not hold a C-cast.

ES.48: Avoid casts

Let’s see what will happen if we screw up the type of system.

// casts.cpp

#include <iostream>

int main(){

  double d = 2;
  auto p = (long*)&d;
  auto q = (long long*)&d;
  std::cout << d << ' ' << *p << ' ' << *q << '\n';
  
}

Neither the result with the Visual Studio compiler

nor the result with the GCC or the clang compiler is promising.

What is bad about the C-cast? You don’t see which cast is performed. If you perform a C-cast, a combination of casts will be applied if necessary. Roughly speaking, a C-cast starts with a static_cast, continues with a const_cast, and finally performs a reinterpret_cast.

Of course, you know how I will continue: explicit is better than implicit.

ES.49: If you must use a cast, use a named cast

Including the GSL, C++ offers eight different named casts. Here are including a short description:

static_cast: conversion between similar types such as pointer types or numeric types
const_cast: adds or removes const or volatile
reinterpret_cast: converts between pointers or between integral types and pointers
dynamic_ cast: converts between polymorph pointers or references in the same class hierarchy
std::move: converts to an rvalue reference
std::forward: converts to an rvalue reference
gsl::narrow_cast: applies a static_cast
gsl::narrow: applies a static_cast

What? std::move and std::forward are casts? Le’s have a closer look at the internals of std::move:

static_cast<std::remove_reference<decltype(arg)>::type&&>(arg)

First, the type of argument arg is determined by decltype(arg). Then all references are removed, and two new references are added. The function std::remove_reference is from the type-traits library. I have already written a few posts to the type-traits library. In the end, we will always get an rvalue reference.

Casting away const is undefined behavior.

ES.50: Don’t cast away `const`

Let me be more specific. Casting away const is undefined behavior if the underlying object, such as constInt is not mutable.

const int constInt = 10;
const int* pToConstInt = &constInt;
 
int* pToInt = const_cast<int*>(pToConstInt);
*pToInt = 12;          // undefined behaviour

If you don’t believe me, there is a footnote in the C standard [ISO/IEC 9899:2011] (subclause 6.7.3, paragraph 4) relevant to the C++ standard: The implementation may place a const object that is not volatile in a read-only region of storage. Moreover, the implementation need not allocate storage for such an object if its address is never used.

Did I mention mutable? mutable is one of the unknown features in C++. mutable allows you to differentiate between bitwise and logical constness. What?

Imagine you want to implement the interface to a telephone book. For simplicity reasons, the entries should be in a std::unordered_map.

// teleBook.cpp

#include <iostream>
#include <string>
#include <unordered_map>

std::unordered_map<std::string, int> getUpdatedTelephoneBook(){
    // generate a new, updated telephone book
    return {{"grimm",123}, {"huber", 456}, {"schmidt", 321}};
}

class TelephoneBook{
public:
    int getNumber(const std::string& name) const {
        auto ent = cache.find(name);
        if(ent != cache.end()){                              
            return ent->second;
        }
        else{
            cache = getUpdatedTelephoneBook();               // (2)
            return cache[name];
        }
    }
private:                                                     // (1)
    std::unordered_map<std::string, int> cache = {{"grimm",123}, {"huber", 456}};
};


int main(){
    
    std::cout << std::endl;
    
    TelephoneBook telBook;                                   // (3)
    
    std::cout << "grimm " << telBook.getNumber("grimm") << std::endl;
    
    std::cout << "schmidt " << telBook.getNumber("schmidt") << std::endl;
    
    std::cout << std::endl;
    
}

My telephone book (1) is tiny. Usually, a telephone book is quite big, and updating it is quite expensive (2). This means updating a printed telephone book will happen only once a year in Germany. From the conceptional view, the inquiries to the teleBook (3) should be const. This is not possible because the unordered_map is modified in the method getNumber. Here is the proof in red ellipses.

The qualifier mutable allows you to differentiate between bitwise and logical constness. The telBook is logical but not bitwise const.

// teleBook.cpp

#include <iostream>
#include <string>
#include <unordered_map>

std::unordered_map<std::string, int> getUpdatedTelephoneBook(){
    // generate a new, updated telephone book
    return {{"grimm",123}, {"huber", 456}, {"schmidt", 321}};
}

class TelephoneBook{
public:
    int getNumber(const std::string& name) const {
        auto ent = cache.find(name);
        if(ent != cache.end()){                              
            return ent->second;
        }
        else{
            cache = getUpdatedTelephoneBook();               // (2)
            return cache[name];
        }
    }
private:                                                     // (1)
    mutable std::unordered_map<std::string, int> cache = {{"grimm",123}, {"huber", 456}};
};


int main(){
    
    std::cout << std::endl;
    
    const TelephoneBook telBook;                             // (3)
    
    std::cout << "grimm " << telBook.getNumber("grimm") << std::endl;
    
    std::cout << "schmidt " << telBook.getNumber("schmidt") << std::endl;
    
    std::cout << std::endl;
    
}

I just added const (3) to the telBook and mutable to the cache (1), and the program behaves as expected.

ES.55: Avoid the need for range checking

I can make it short. By using the range-based for-loop or algorithms of the STL, there is no need to check the range.

std::array<int, 10> arr = {5, 7, 4, 2, 8, 6, 1, 9, 0, 3}; 
std::sort(arr.begin(), arr.end());
for (auto a : arr) {
    std::cout << a << " ";
}   
// 0 1 2 3 4 5 6 7 8 9

What’s next?

In the next post on expressions, I will write about std::move, new and delete, and slicing. Slicing is probably one of the darkest corners of C++. So, stay tuned.

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