Towards Modern C++: Compile Time Programming

In the realm of C++, Compile Time Programming is a potent domain where the language's evolution has brought forth formidable tools. This article delves into several powerful aspects that elevate programming capabilities at the compilation stage.

Variadic Templates: Unleashing Flexible Functionality

Evolution from C to Modern C++

The concept of variadic functions has its roots in C programming, lacking robust type-safety checks. In C++, the ... operator facilitated functions to accept multiple arguments, often at the cost of type safety. Modern C++ addressed this with variadic templates, introducing a paradigm shift in handling multiple parameters:

template<typename... Args>
void func(Args... args){  }

Pack Expansion: Unveiling the Power

Pack expansion, denoted by the ... operator, enables the compiler to replace a function parameter pack with a corresponding list of arguments. However, context matters:

template<typename... Args>
void func(Args... args){ 
    otherFunc(args...); // Call to otherFunc(arg1, arg2, … );
}

The above requires a context where list of arguments are expected.

std::cout<< args…<< std::endl; // ERROR – No expansion possible
auto myTuple = make_tuple(args…); //OK

For variadic template to do anything useful, it must process the arguments. The way it is done, is via providing an extra non-variadic argument, which represents the popped element from the list.

template< typename T >
void func(T t){ 
    //Base case for recursion
}

template< typename T, typename… Args>
void func(T t, Args… args){ 
    //Do some operation
    func(args…); //Recursive call to func()
}

Variadic templates find valuable application in scenarios like loggers, offering a streamlined approach to continuously log information during event processing.

Static Assertion with static_assert

The static_assert empowers compile-time checks, ensuring specific conditions hold true before the program proceeds. If the condition provided to static_assert evaluates to false, it generates a compilation error with an associated error message.

Here's the basic syntax of static_assert:

static_assert(condition, message);

• condition: This is a compile-time constant expression that the compiler evaluates. If it's false, a compilation error is triggered.
• message: This optional argument is a string literal that serves as the error message displayed if the assertion fails. It helps programmers understand why the assertion failed.

For instance, consider the following example:

static_assert(sizeof(int) == 8, "System must be configured with 64 bit compiler for this operation to work!!");

static_assert is valuable for enforcing constraints and ensuring that critical conditions are met at compile time, reducing the likelihood of runtime errors by catching potential issues during the compilation phase itself. Because of this reason, they find their application in developing unit test suites.

Template Bloating Reduction

Template bloating refers to a situation in C++ where the excessive instantiation of templates leads to larger executable sizes. This happens when a template is used across multiple translation units (source files), resulting in the creation of multiple copies of the template's code for each different set of template arguments used.

In C++11, one method to mitigate template bloating is by using the extern template declaration. This declaration helps in explicitly instructing the compiler to instantiate the template in a single translation unit (usually a source file) and to treat it as if it were declared externally in other translation units.

Here's an example to demonstrate template bloating.

Consider a template function defined as below in myheader.h.

// myheader.h
template<typename T>
void func(arg<T>){
     // Do something
}

If say, there are 300 source files all of which use this template function with string type parameter, then the final executable size will be higher given that for each file compiler will have to link the same template code again and again. This causes bloating of final executable file resulting in an increased size.

// source1.cpp
#include<"myheader.h">
func(string1);
...
// source300.cpp
#include<"myheader.h">
func(string300);

To avoid the above, we use extern template feature of C++11. In that, one of the files will instantiate the potential candidate responsible for future bloating, in our case the function func with string type parameters.

//mainInstantiationFile.cpp
template void func(const std::string& arg);

In myheader.h, we define the external linkage with above instantiation.

// myheader.h
template<typename T>
void func(arg<T>){
     // Do something
}

extern template void func(const std::string& arg);

Now, we can use this external template across all the source files

//source1.cpp
#include<myheader.h>
func(string1);    // invoke the void func(const std::string& arg) function

//source300.cpp
#include<myheader.h>
func(string300);    // invoke the void func(const std::string& arg) function

This technique prevents the unnecessary generation of multiple copies of the same template code for different types across different translation units, thereby reducing template bloat and the resulting increase in executable size.

Embracing constexpr

constexpr stands for "constant expression" and is used to indicate that an expression or function must be evaluated at compile time. It enables computations during compilation, providing the compiler with information about values that are known in advance.

• const restricts modifications to variables or function arguments.
• constexpr guarantees that compiler computes constant values at compile time, enabling optimization.

constexpr int square(int x) {
    return x * x;
}

int main() {
    constexpr int result = square(5);  // Computed at compile time
    // ...
}

C++11 allows constexpr methods to only include a return statement. With C++14, their capabilities expanded to include multiple statements and variable declarations, barring certain operations like dynamic memory allocation and exceptions.

Automatic Type Deduction

auto is a keyword introduced in C++11, facilitating automatic type deduction during variable declaration. It allows the compiler to infer the type of a variable from its initializer.

Benefits of auto:

• Improved Readability: auto simplifies code by reducing verbosity, especially in cases where types can be complex or lengthy.
• Flexibility and Maintenance: It adapts to changes in types without requiring manual adjustments, promoting code flexibility and easing maintenance efforts.

auto value = 10;  // Compiler deduces type as int
auto name = "C++";  // Compiler deduces type as const char*

The synergy of constexpr and auto can provide significant advantage in compile time evaluation of expressions.

constexpr auto calculateArea(int length, int width) {
    return length * width;
}

int main() {
    constexpr auto area = calculateArea(5, 10);  // Computed at compile time, type inferred
    // ...
}

Advantage of using auto and constexpr together could be:

• Compile-Time Efficiency: Utilizing constexpr for computations combined with auto for type deduction enhances both efficiency and readability.
• Enhanced Metaprogramming: The combination enables powerful metaprogramming capabilities, allowing compile-time evaluation and simplified code structures.

Conclusion

Compile time programming in C++ opens doors to a world where efficiency meets flexibility. Embracing these powerful features empowers developers to create robust, optimized, and maintainable codebases, transforming the way we perceive and craft software solutions.