C++ Core Guidelines: Rules to Statements and Arithmetic

Today, I will write about the remaining rules to statements and the arithmetic rules. If you don't follow the arithmetic rules, undefined behaviour may kick in.

 

mathematics 678969 640 Four rules to statements are left. Here are they:

 The first rule is quite obvious.

ES.84: Don’t (try to) declare a local variable with no name

Declaring a local variable without a name has no effect. With the final semicolon, the variable will go out of scope.

void f()
{
    lock<mutex>{mx};   // Bad
    // critical region
}

 

Typically, the optimiser can remove the creation of a temporary, if it will not change the observable behaviour of the program. This is the so-called as-if rule. To put is the other way around. If the constructor has observable behaviour such as modifying the global state of the program, the optimiser is not allowed to remove the creation of the temporary.

ES.85: Make empty statements visible

To be honest, I don't get the reason for this rule. Why do you want to write empty statements? For me, both examples are just bad.

for (i = 0; i < max; ++i);   // BAD: the empty statement is easily overlooked
v[i] = f(v[i]);

for (auto x : v) {           // better
    // nothing
}
v[i] = f(v[i]);

 

ES.86: Avoid modifying loop control variables inside the body of raw for-loops

Ok. That is from two perspectives really a very bad practice. First, you should avoid to write raw loops and use the algorithms of the Standard Template Library. Second, you should not modify the control variable inside the for-loop. Here is the bad practice. 

for (int i = 0; i < 10; ++i) {
    //
    if (/* something */) ++i; // BAD
    //
}

bool skip = false;
for (int i = 0; i < 10; ++i) {
    if (skip) { skip = false; continue; }
    //
    if (/* something */) skip = true;  // Better: using two variable for two concepts.
    //
}

 

What makes it difficult for me to reason in particular about the second for-loop is that this are under the hood two nested dependent loops.  

ES.87: Don’t add redundant == or != to conditions

 I'm guilty. In my first years as professional C++ developer I often used redundant == or != in conditions. Of course, this changed in the meantime.

// p is not a nullptr
if (p) { ... }            // good
if (p != nullptr) { ... } // redundant 

// p is a nullptr
if (!p) { ... }           // good
if (p == 0) { ... }       // redundant 

for (string s; cin >> s;)  // the istream operator returns bool
v.push_back(s);

 

These were the rules to statements. Let's continue with the arithmetic rules. Here are the first seven.

Honestly, there is often not so much for me to add to this rules. For the sake of completeness (and importance), I will briefly present the rules.

ES.100: Don’t mix signed and unsigned arithmetic

If you mix signed and unsigned arithmetic, you will not get the expected result. 

#include <iostream>

int main(){

  int x = -3;
  unsigned int y = 7;

  std::cout << x - y << std::endl;  // 4294967286
  std::cout << x + y << std::endl;  // 4
  std::cout << x * y << std::endl;  // 4294967275
  std::cout << x / y << std::endl;  // 613566756
  
}

 

 GCC, Clang, and the Microsoft Compiler produced the same results.

ES.101: Use unsigned types for bit manipulation

The reason for the rules is quite simple. Bitwise operations on signed types are implementation-defined. 

ES.102: Use signed types for arithmetic

First, you should make arithmetic with signed types. Second, you should not mix signed and unsigned arithmetic. If not, the results may surprise you.

 

#include <iostream>

template<typename T, typename T2>
T subtract(T x, T2 y){
    return x - y;
}

int main(){
    
    int s = 5;
    unsigned int us = 5;
    std::cout << subtract(s, 7) << '\n';       // -2
    std::cout << subtract(us, 7u) << '\n';     // 4294967294
    std::cout << subtract(s, 7u) << '\n';      // -2
    std::cout << subtract(us, 7) << '\n';      // 4294967294
    std::cout << subtract(s, us + 2) << '\n';  // -2
    std::cout << subtract(us, s + 2) << '\n';  // 4294967294

    
}

ES.103: Don’t overflow and ES.104: Don’t underflow

Let me combine both rules. The effect of an overflow or an underflow is the same: memory corruption and undefined behaviour. Let's make a simple test with an int array. How long will the following program run?

// overUnderflow.cpp

#include <cstddef>
#include <iostream>

int main(){
    
    int a[0];
    int n{};

    while (true){
        if (!(n % 100)){
            std::cout << "a[" << n << "] = " << a[n] << ", a[" << -n << "] = " << a[-n] << "\n";
        }
        a[n] = n;
        a[-n] = -n;
        ++n;
    }
    
}

 

Disturbing long. The program writes each 100th array entry to std::cout. 

overUnderflow

ES.105: Don't divide by zero

If you want to have a crash you should divide by zero. Diving by zero may be fine in a logical expression.

bool res = false and (1/0);

 

Because the result of the expression (1/0) is not necessary for the overall result, it will not be evaluated. This technique is called short circuit evaluation and is a special case of lazy evaluation. 

ES.106: Don’t try to avoid negative values by using unsigned

Don't use an unsigned type if you want to avoid negative values. The consequences may be serious. The behaviour of arithmetic will change and you are open to errors including signed/unsigned arithmetic.

Here are two examples of the Guidelines, intermixing signed/unsigned arithmetic.

unsigned int u1 = -2;   // Valid: the value of u1 is 4294967294
int i1 = -2;
unsigned int u2 = i1;   // Valid: the value of u2 is 4294967294
int i2 = u2;            // Valid: the value of i2 is -2


unsigned area(unsigned height, unsigned width) { return height*width; } 
// ...
int height;
cin >> height;
auto a = area(height, 2);   // if the input is -2 a becomes 4294967292

 

As the Guidelines stated there is an interesting relation. When you assign a -1 to an unsigned int, you will become the largest unsigned int.

Now to the more interesting case. The behaviour of arithmetic will differ between signed and unsigned types.

Let's start with a simple program. 

 

// modulo.cpp

#include <cstddef>
#include <iostream>

int main(){
    
    std::cout << std::endl;
    
    unsigned int max{100000};    
    unsigned short x{0};                 // (2)
    std::size_t count{0};
    while (x < max && count < 20){
        std::cout << x << " ";           
        x += 10000;                      // (1)
        ++count;
    }
    
    std::cout << "\n\n";
}

 

The key point of the program is that the successive addition to x in line (1) will not trigger an overflow but a modulo operation if the value range of x ends. The reason is that x is of type unsigned short (2). 

 

// overflow.cpp

#include <cstddef>
#include <iostream>

int main(){
    
    std::cout << std::endl;
    
    int max{100000};    
    short x{0};                     // (2)
    std::size_t count{0};
    while (x < max && count < 20){
        std::cout << x << " ";
        x += 10000;                  // (1)
        ++count;
    }
    
    std::cout << "\n\n";
}

 

I made a small change to the program modulo.cpp such that x (2) becomes a signed type. The same addition will now trigger an overflow.

I marked the key points with red circles in the screenshot.

 ModuloOverflow

Now, I have a burning question: How can I detect an overflow? Quite easy. Replace the erroneous assignment  x += 1000; with an  expression using curly braces: x = {x + 1000};. The difference is that the compiler checks narrowing conversions and, therefore, detects the overflow. Here is the output from GCC. 

narrowingConversion

Sure the expressions (x += 1000) and (x  = {x + 1000}) are from a performance perspective not the same. The second one could create a temporary for x + 1000. But in this case, the optimiser did a great job and both expressions were under the hood the same. 

What's next?

I'm nearly done with the arithmetic rules. This means in the next post I will continue my journey with the rules to performance.

 

 

 

 

Thanks a lot to my Patreon Supporters: Eric Pederson, Paul Baxter, Carlos Gomes Martinho, and SAI RAGHAVENDRA PRASAD POOSA.

 

 

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