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1 Introduction
In C++14, a new feature - variable templates (Variable Templates) was introduced. This feature has a wide range of applications in programming, especially when writing generic code, it can provide a more concise and intuitive way to define and use template variables.
1.1 Introduction and significance of variable templates
Before C++14, we can use templates to define types and functions, but for variables, we can only define them in template classes or template functions. This can lead to redundant and complex code in some cases. For example, if we want to define a constant for each type, we may need to define a template class, and then define this constant in this class . Not only does this make the code redundant, but it also makes using this constant unintuitive.
To solve this problem, C++14 introduces variable templates. Variable templates allow us to define template variables directly without wrapping them in template classes or template functions. This allows us to define and use template variables more intuitively and concisely.
The introduction of variable templates greatly improves the expressiveness of C++, allowing us to write generic code more concisely and intuitively. It has a wide range of applications in many fields, such as mathematical calculations, data structure design, template library development and other fields, you can see it.
Below, we will use some specific examples to deeply understand the definition and use of variable templates, as well as its application in actual programming.
// 定义一个变量模板
template<typename T>
constexpr T pi = T(3.1415926535897932385);
// 使用变量模板
double circumference(double radius) {
return 2 * pi<double> * radius;
}
In this example, we define a variable template pi, and then use this template in the function circumference. This is much more intuitive than defining and using constants in template classes or template functions.
2. Basic concepts of variable templates
In C++14, we can define a template whose instantiation result is a variable instead of a type or function. This is the so-called variable template (Variable Templates). In this chapter, we'll dig into the basic concepts of variable templates, including their definition, instantiation, and relationship to type deduction.
2.1 Definition and instantiation of variable templates
A variable template is defined in a similar way to a function template or a class template, except that it defines a variable. Here is a simple example:
template<typename T>
constexpr T pi = T(3.1415926535897932385);
In this example, pi is a variable template that can be used for any type T. When you use pi<double>, you get a π value of type double. Again, pi<int>will give you a π value of type int (in this case it will be truncated to 3).
Variable templates can also define static variables:
template<typename T>
static constexpr T pi = T(3.1415926535897932385);
2.2 Intra-class template variable definition
struct limits {
template<typename T>
static T min; // declaration of a static data member template
};
template<typename T>
T limits::min = {
}; // definition of a static data member template
Its behavior is consistent with static variables. Non-const static variables need to be defined outside the class.
If they are const, they can be defined inside the class:
struct limits {
template<typename T>
static const T min{
}; // definition of a static data member template
};
2.2 Variable templates and type deduction
In C++14, we can use the auto keyword for type deduction, which is very useful when dealing with variable templates. For example, we can define a variable template and use the auto keyword to automatically deduce its type:
template<typename T>
constexpr T pi = T(3.1415926535897932385);
auto pi_double = pi<double>788u,; // 类型为double
auto pi_int = pi<int>; // 类型为int
In this example, the types of pi_double and pi_int are both automatically deduced by the auto keyword. This avoids the need to explicitly specify the type when writing code, making the code more concise.
2.3 Application of variable templates
Variable templates have many applications in actual programming. For example, we can use variable templates to define a set of related constants, as follows:
template<typename T>
constexpr T zero = T(0);
template<typename T>
constexpr T one = T(1);
template<typename T>
constexpr T two = T(2);
In this example, we define a set of constants zero, one, and two that can be used for any type T. In this way, we can use zero<int>, one<double>, etc. to get different types of constant values.
The above is the basic concept and application of variable templates. In the next chapter, we will explore the advanced applications of variable templates and their role in metaprogramming.
3. Advanced application of variable templates
In this chapter, we will delve into the advanced applications of C++14 variable templates, including how to use variable templates to define constants, how to use them with non-type template parameters, and how to perform template specialization.
3.1 Using variable templates to define constants
A common use of variable templates is to define constants. For example, we can define a variable template representing the mathematical constant π:
template<typename T>
constexpr T Pi = T(3.1415926535897932385);
This variable template can be used for any type T. When you use Pi<double>, you get a π value of type double. Again, Pi<int>will give you a π value of type int (in this case it will be truncated to 3).
The advantage of this approach is that we can provide the same constant for different types without needing to define a separate constant for each type. This makes the code cleaner and easier to maintain.
3.2 Variable templates and non-type template parameters
A variable template can have multiple template parameters, which can be types or non-types. For example, we can define a variable template representing a fixed-size array:
template<typename T, int N>
T array[N];
Then, you can use array<int, 10>to get an array of 10 integers.
The advantage of this approach is that we can create arrays for different types and sizes without needing to define a separate array for each type and size. This makes the code more flexible and easier to maintain.
3.3 Variable templates and template specialization
Template specialization is an important feature of the C++ template system, which allows us to provide special template definitions for specific types or values. Variant templates can also be specialized. For example, we could provide a specialization for the Pi template above that handles int types:
template<>
constexpr int Pi<int> = 3;
This specialization will be used when T is int instead of the original template definition. This allows us to provide special behavior for certain types without affecting other types.
In the table below we summarize the main features of the advanced application of variable templates presented in this chapter:
4. Variable templates and metaprogramming
Metaprogramming is a programming technique that performs computations at compile time rather than at run time. C++'s template system provides powerful metaprogramming capabilities, and variable templates (Variable Templates) further expand this capability.
4.1 The role of variable templates in metaprogramming
The main role of variable templates in metaprogramming is to provide a way to define compile-time constants (Compile-time Constants). These constants can be calculated and manipulated at compile time without waiting until run time. This is very useful for optimizing code and reducing runtime overhead.
For example, we can define a variable template to compute factorial:
template<int N>
constexpr int factorial = N * factorial<N - 1>;
template<>
constexpr int factorial<0> = 1;
The factorial template calculates the result at compile time, so factorial<5> is a constant in the code with a value of 120.
4.2 Variable templates and compile-time evaluation
Variable templates, together with constexpr functions, enable powerful compile-time computations. A constexpr function can compute its results at compile time, and a variable template can store those results.
For example, we can define a variable template to store the values of the Fibonacci sequence:
constexpr int fibonacci(int n) {
return (n <= 1) ? n : (fibonacci(n - 1) + fibonacci(n - 2));
}
template<int N>
constexpr int fib = fibonacci(N);
In this example, fib<10> is a constant in the code with a value of 55.
4.3 Variable templates and type metaprogramming
Variable templates can be used not only for value metaprogramming, but also for type metaprogramming. Type metaprogramming is a technique for manipulating types using templates, which can be used to implement type-level computations and decisions.
For example, we can define a variable template to determine the greater of two types:
template<typename T, typename U>
constexpr bool is_t_larger = (sizeof(T) > sizeof(U));
In this example, is_t_larger<int, double> is a constant in the code, and its value is false, because the size of double is usually larger than int.
These are just some examples of the use of variable templates in metaprogramming, in fact, their use is much more than these. Variable templates provide a powerful tool that can help us write more efficient and flexible code.
5. Practical application cases of variable templates
In this chapter, we will demonstrate the power of C++14 variable templates through some practical application cases. As we will see, variable templates can not only make our code more concise, but also improve its performance and readability.
5.1 Using variable templates to optimize mathematical calculations
Let's start with a simple example, assuming we need to use the mathematical constant π frequently in our code. In the absence of variable templates, we might define a constant for each required type, as follows:
const float pi_float = 3.1415926535897932385f;
const double pi_double = 3.1415926535897932385;
const long double pi_long_double = 3.1415926535897932385L;
This approach is not only redundant, but also difficult to maintain. If we need to add a new type, we need to define a new constant. And with variable templates, we can simplify this process:
template<typename T>
constexpr T pi = T(3.1415926535897932385);
Now, we can use pi<float>、pi<double>和pi<long double>to get the value of π we need. This makes our code cleaner and easier to maintain.
5.2 Application of variable templates in data structure design
Variable templates can also play an important role in designing data structures. For example, we can use variable templates to define a fixed-size array. This is especially useful in embedded programming, where we often need precise control over memory usage.
template<typename T, int N>
T array[N];
Then, we can use array<int, 10> to create an array of 10 integers. This is more efficient than using dynamically allocated arrays because it avoids the overhead of memory allocation and deallocation.
5.3 Application of variable templates in template library
Many modern C++ libraries use variable templates. For example, std::tuple_size in the standard library is a variable template that returns the size of a tuple. This is a very powerful tool because it allows us to get the size of the tuple at compile time, which is very useful for metaprogramming.
#include <tuple>
std::tuple<int, double, char> t;
constexpr auto size = std::tuple_size<decltype(t)>::value; // size is 3
In this example, std::tuple_size<decltype(t)>::value is a compile-time constant whose value is equal to the size of the tuple t. This allows us to do some optimizations at compile time, for example, we can use this value to define a fixed-size array.
The above are some examples of variable templates in practical applications. Through these examples, we can see that variable templates are a very powerful tool, which can make our code more concise and easier to maintain, and can also improve the performance of the code.
6. Limitations and challenges of variable templates
In the previous chapters, we have seen the power of variable templates, however, any technology has its limitations and challenges, and variable templates are no exception. In this chapter, we will explore these problems in depth and provide some solutions.
6.1 Limitations of variable templates
6.1.1 Restrictions on template parameters
First, the template parameter of a variable template must be a constant expression. This means you cannot use runtime values as template parameters. For example, the following code is invalid:
template<int N>
int array[N];
int size = 10;
array<size> myArray; // 错误：size不是一个常量表达式这是因为模板是在编译时实例化的，所以模板参数必须在编译时就已知。
6.2 How to solve the challenge of variable templates
Although variable templates have the above limitations, we can work around them with some tricks.
6.2.1 Using constexpr functions
For the limitation of template parameters, we can use the constexpr function to solve it. A constexpr function is a special kind of function whose result can be computed at compile time. Therefore, we can use constexpr functions to generate template parameters. For example:
constexpr int getSize() {
return 10;
}
template<int N>
int array[N];
array<getSize()> myArray; // 正确：getSize()是一个constexpr函数
6.2.2 Using default initialization
For the initialization problem of variable templates, we can use default initialization to solve it. Default initialization gives a variable a default initial value. For example, for integer types, its default initial value is 0. Therefore, we can define variable templates like this:
template<typename T>
T value = T(); // 正确：使用默认初始化
In this example, T() will generate a default value of type T. For example, if T is an int, then T() is 0.
The above are some limitations and solutions of variable templates. In actual programming, we need to choose the appropriate method according to the specific situation.
7. The Future of Variable Templates
In this chapter, we explore the progress of variable templates in C++17 and C++20, and the future direction of variable templates.
7.1 The evolution of variable templates in C++17 and C++20
C++17 and C++20 introduce some new features in template programming that have a direct or indirect impact on the use of variable templates.
7.1.1 Variable templates in C++17
In C++17, the if constexpr (if constexpr) statement was introduced, which provides more flexibility for the use of variable templates. if constexpr allows conditional judgment at compile time, which is very useful for metaprogramming and compile-time calculations. For example, we can use if constexpr to define a variable template that is only valid on certain types:
template<typename T>
constexpr auto is_integral = std::is_integral_v<T>;
template<typename T>
constexpr auto value = if constexpr (is_integral<T>) T(1) else T(0.0);
In this example, the value of the value variable template depends on whether the template parameter T is of integer type. The value is 1 if T is an integer type, value<T>and 0.0 otherwise.
7.1.2 Variable templates in C++20
C++20 introduces Concepts, a powerful new feature that allows us to impose more fine-grained constraints on template parameters. This provides greater flexibility and security for the definition and use of variable templates. For example, we can define a variable template that accepts only integer types:
template<std::integral T>
constexpr T value = T(1);
In this example, if we try to instantiate value with a non-integer type, the compiler will throw an error.
7.2 Future development direction of variable templates
Variant templates are an important part of the C++ template system, which may see more development in future C++ standards. Here are some possible directions to go:
More powerful compile-time computations: As compiler technology advances, we're likely to see more powerful compile-time computations, which will allow variable templates to handle more complex computations and data structures.
Finer template parameter constraints: As the concept develops, we may see finer template parameter constraints, which will make the definition and use of variable templates safer and more flexible.
Better template error diagnosis: Diagnosis of template errors has always been a challenge in C++. Future C++ standards may provide better tools and techniques to help programmers understand and resolve template errors.
The above are some guesses about the possible future development direction of variable templates, and the actual development may be different. In any case, variable templates, as an important part of the C++ template system, will continue to play an important role in the development of C++.