Although rule T.11 states: Whenever possible use standard concepts you sometimes have to define your concepts. This post gives you rules to do it.
The C++ core guidelines has nine rules for defining concepts. Seven of them have content. Here are the first four for today.
Let's see how to define concepts
This rule is quite obvious but what does meaningful semantic mean. Meaningful semantics are not just simple constraints such as has_plus
but concepts such as Number,
Range,
or InputIterator.
For example, the following concept Addable
requires has_plus
and is, therefore, also fulfilled by a string.
template<typename T>
concept Addable = has_plus<T>; // bad; insufficient
template<Addable N> auto algo(const N& a, const N& b) // use two numbers
{
// ...
return a + b;
}
int x = 7;
int y = 9;
auto z = algo(x, y); // z = 16
string xx = "7";
string yy = "9";
auto zz = algo(xx, yy); // zz = "79"
I assume this was not your intention because the function template algo
should accept arguments which model numbers and not just addable. The solution is quite simple. Define and use a concept Number
with a meaningful semantic.
template<typename T>
// The operators +, , *, and / for a number
// are assumed to follow the usual mathematical rules
concept Number = has_plus<T>
&& has_minus<T>
&& has_multiply<T>
&& has_divide<T>;
template<Number N> auto algo(const N& a, const N& b)
{
// ...
return a + b;
}
Now the invocation of algo
with a string would give an error. The next rule is a particular case of this rule.
First of all, what is a complete set of operations? Here are two complete sets for Arithmetic
and Comparable.
 Arithmetic:
+, , *, /, +=, =, *=, /=
 Comparable:
<, >, <=, >=, ==, !=
Do you want for what the acronym POLA stands for? It stands for Principle Of Least Astonishment. You can quite easily break this principle of good software design if you implement just a partial set of operations.
Here is a very promising example from the guidelines. The concept Minimal
in this case supports==, <
and +.
void f(const Minimal& x, const Minimal& y)
{
if (!(x == y)) { /* ... */ } // OK
if (x != y) { /* ... */ } // surprise! error
while (!(x < y)) { /* ... */ } // OK
while (x >= y) { /* ... */ } // surprise! error
x = x + y; // OK
x += y; // surprise! error
}
First of all: What is an axiom? Here is my definition from Wikipedia:
 An axiom or postulate is a statement that is taken to be true, to serve as a premise or starting point for further reasoning and arguments.
Because C++ does not support axioms, you have to express them with comments. If C++ does support them in the future, you can remove the comment symbol //
in front of the axiom in the following example.
template<typename T>
// axiom(T a, T b) { a + b == b + a; a  a == 0; a * (b + c) == a * b + a * c; /*...*/ }
concept Number = requires(T a, T b) {
{a + b} > T;
{a  b} > T;
{a * b} > T;
{a / b} > T;
}
The axiom means in this case that the number follow the mathematical rules. In contrast, the concept requires that a Number has to support the binary operations +, , *,
and /
and that the result is convertible to T. T
is the type of the arguments.
If two concepts have the same requirements, they are logically equivalent. This means the compiler can't distinguish them and, therefore, may not choose automatically the correct one during overload resolution.
To make the rule clear, here is a simplified version of the concept BidirectionalIterator
and the refined concept RandomAccessIterator.
template<typename I>
concept bool BidirectionalIterator = ForwardIterator<I> &&
requires(I iter){
iter;
iter;
}
template<typename I>
concept bool RandomAccessIterator = BidirectionalIterator<I> &&
Integer<N> &&
requires(I iter, I iter2, N n){
iter += n; // increment or decrement an iterator
iter = n;
n + iter; // return a temp iterator
iter + n;
iter  n;
iter[n]; // access the element
iter1  iter2; // subtract two iterators
iter1 < iter2; // compare two iterators
iter1 <= iter2;
iter1 > iter2;
iter1 >= iter2;
}
std::advance(i, n)
increments a given iterator i by n elements. Depending on the value of n
, the iterator is
incremented or decremented. When the iterator i
is a bidirectional iterator, std::advance
has to step n
times one element forwards or backwards. But when the iterator i is a random access iterator, just n
is added to the iterator.
template<BidirectionalIterator I>
void advance(I& iter, int n){...}
template<RandomAccessIterator I>
void advance(I& iter, int n){...}
std::list<int> lst{1, 2, 3, 4, 5, 6, 7, 8, 9};
std::list<int>::iterator listIt = lst.begin();
std::advance(listIt, 2); // BidirectionalIterator
std::vector<int> vec{1, 2, 3, 4, 5, 6, 7, 8, 9};
std::vector<int>::iterator vecIt = vec.begin();
std::advance(vecIt, 2); // RandomAccessIterator
In case of the std::vector<int>, vec.begin()
returns an random access iterator and, therefore, the fast variant of std::advance
is used.
Each container of the STL creates an iterator specific to its structure. Here is the overview:
What's next?
Three rules to the definition of concepts are left. In particular, the next rule "T.24: Use tag classes or traits to differentiate concepts that differ only in semantics." sound quite interesting. Let's see in the next post what a tag class or traits class is.
Thanks a lot to my Patreon Supporters: Eric Pederson, Paul Baxter, Meeting C++, Matt Braun, Avi Lachmish, Roman Postanciuc, Venkata Ramesh Gudpati, Tobias Zindl, Mielo, and Dilettant.
Thanks in particular to:
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