Template Metaprogramming – Hybrid Programming
First of all, hybrid programming is not an official term. I created it to emphasize an exciting aspect of templates. The difference between function arguments and template arguments.
I ended my last post, “Template Metaprogramming – How it Works” with a riddle. Here is the context for the riddle.
The Riddle
The function power
and Power
calculate the pow(2, 10). power
is executed at run time and Power
at compile time.
// power.cpp #include <iostream> int power(int m, int n) { int r = 1; for(int k = 1; k <= n; ++k) r *= m; return r; } template<int m, int n> struct Power { static int const value = m * Power<m, n-1>::value; }; template<int m> struct Power<m, 0> { static int const value = 1; }; int main() { std::cout << '\n'; std::cout << "power(2, 10)= " << power(2, 10) << '\n'; std::cout << "Power<2,10>::value= " << Power<2, 10>::value << '\n'; std::cout << '\n'; }
If you want more details about both functions, read my previous post, “Template Metaprogramming – How it Works“.
So far, so good, but what is happening in the following example?
// powerHybrid.cpp #include <iostream> template<int n> int Power(int m){ return m * Power<n-1>(m); } template<> int Power<0>(int m){ return 1; } int main() { std::cout << '\n'; std::cout << "Power<0>(10): " << Power<0>(20) << '\n'; std::cout << "Power<1>(10): " << Power<1>(10) << '\n'; std::cout << "Power<2>(10): " << Power<2>(10) << '\n'; std::cout << '\n'; }
As expected, Power
does its job.
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Here is the riddle in short one more: Is Power
a function or a metafunction?
Hybrid Programming
To make it short.
The calls Power<0>(10)
, Power<1>(10)
, and Power<2>(10)
use sharp and round brackets and calculate 10 to the power of 0, 1, and 2. This means 0, 1, and 2 are compile-time arguments, and 10 is a run-time argument. To say it differently: Power is, at the same time, a function and a metafunction. Let me elaborate more on this point.
Power at Run Time
First, I can instantiate Power
for 2, give it the name Power2
and use it in a for-loop.
// powerHybridRuntime.cpp #include <iostream> template<int n> int Power(int m){ return m * Power<n-1>(m); } template<> int Power<0>(int m){ return 1; } int main() { std::cout << '\n'; auto Power2of = Power<2>; for (int i = 0; i <= 20; ++i) { std::cout << "Power2of(" << i << ")= " << Power2of(i) << '\n'; } std::cout << '\n'; }
Power2o
f enables it to calculate the squares of 0 … 20 at run time.
You cannot invoke Power
with different template arguments in the for-loop. Template instantiation requires a constant expression. To make it short: The following use of Power fails with a compile-time error that “the value of 'i' is not usable in a constant expression
“.
for (int i = 0; i <= 20; ++i) {
std::cout << “Power<“ << i << “>(2)= “ << Power<i>(2) << ‘\n’;
}
Honestly, there is a more interesting difference between a function and a metafunction.
Power at Compile Time
When you study the previous program powerHybrid.cpp
in C++ Insights, you see that each usage of Power with a different template argument creates a new type.
This means that the invocation Power<2>(10)
causes the recursive template instantiation for Power<1>(10)
, and Power<0>(10)
. Here is the output of C++ Insights.
To sum up my observation. Each template instantiation creates a new type.
Creating New Types
When you use a template such as Power
, std::vector
, or std::array
, you can invoke it with two kinds of arguments: function arguments and template arguments. The function arguments go into the round brackets (( ... )
) and the template arguments go into the sharp brackets (<...>
). The template arguments create new types. Or, to put it the other way around. You can parameterize templates in two ways: at compile time with sharp brackets (<...>
). and at run time with round brackets (( ... )
.
auto res1 = Power<2>(10); // (1) auto res2 = Power<2>(11); // (2) auto rest3 = Power<3>(10); // (3) std::vector<int> myVec1(10); // (1) std::vector<int> myVec2(10, 5); // (2) std::vector<double> myDouble(5); // (3) std::array<int, 3> myArray1{ 1, 2, 3}; // (1) std::array<int, 3> myArray2{ 1, 2, 3}; // (2) std::array<double, 3> myArray3{ 1.1, 2.2, 3.3}; // (3)
- (1) creates a new
Power
instance,std::vector
of length 10, or astd::array
with three elements - (2) reuses the already created types in the previous lines (1)
- (3) creates a new type
A few of my German readers already pointed it out. My metafunction Power has a significant flaw.
The Big Flaw
I get undefined behavior when I instantiate with a negative or too-big number.
Power<-1>(10)
causes an infinite template instantiation because the boundary condition Power<0>(10) does not apply.Power<200>(10)
causes anint
overflow.
The first issues can be fixed by using a static_assert
inside the Power
template: static_assert(n >= 0, "exponent must be >= 0");.
There is no simple solution for the second issue.
// powerHybridRuntimeOverflow.cpp #include <iostream> template<int n> int Power(int m){ return m * Power<n-1>(m); } template<> int Power<0>(int m){ return 1; } int main() { std::cout << '\n'; auto Power10of = Power<10>; for (int i = 0; i <= 20; ++i) { std::cout << "Power10of(" << i << ")= " << Power10of(i) << '\n'; } std::cout << '\n'; }
The overflow starts with Power10of(9). pow(9, 10) is
3,486,784,40
My Disclaimer
At the end of these three posts, “Template Metaprogramming – How it All Started“, “Template Metaprogramming – How it Works” about template metaprogramming, I have to make a disclaimer. I don’t want you to use templates to program at compile time. Most of the time, constexpr
(C++11) or consteval
(C++20 is the better choice.
I explained template metaprogramming for two reasons.
- Template metaprogramming helps you better understand templates and the process of template instantiation.
- The type-traits library applies the idea and uses the conventions of template metaprogramming.
What’s next?
In my next post, I will write about the type-traits library. The type-traits library (C++11) is template metaprogramming in a beautiful guise.
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