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Review of Manning's Functional Programming in C++

Last year I reviewed the pre-print manuscript of Manning's Functional Programming in C++ written by Ivan Čukić.
I really enjoyed reading the book. I enthusiastically support that the book
Offers precise, easy-to-understand, and engaging explanations of functional concepts.Who is this book for This book expects a reasonable working knowledge of C++, its modern syntax, and semantics from the readers. Therefore, reading this book might require a companion book for C++ beginners. I think that’s fair because FP is an advanced topic. C++ is getting more and more powerful day by day. While there are many FP topics that could be discussed in such a book, I like the practicality of the topics selected in this book.

Here's the table of contents at a glance. This is a solid coverage of functional programming concepts to get a determined programmer going from zero-to-sixty in a matter of weeks. Others have shared their thoughts on this book as well. See Rangarajan Krishnamoorthy&…
Recent posts

Unit Testing C++ Templates and Mock Injection Using Traits

Unit testing your template code comes up from time to time. (You test your templates, right?) Some templates are easy to test. No others. Sometimes it's not clear how to about injecting mock code into the template code that's under test. I've seen several reasons why code injection becomes challenging.

Here I've outlined some examples below with roughly increasing code injection difficulty. Template accepts a type argument and an object of the same type by reference in constructorTemplate accepts a type argument. Makes a copy of the constructor argument or simply does not take oneTemplate accepts a type argument and instantiates multiple interrelated templates without virtual functions Lets start with the easy ones.
Template accepts a type argument and an object of the same type by reference in constructor This one appears straight-forward because the unit test simply instantiates the template under test with a mock type. Some assertion might be tested in the mock c…

Simple Template Currying

Currying is the technique of transforming a function that takes multiple arguments in such a way that it can be called as a chain of functions, each with a single argument. I've discussed Currying on this blog previously in Fun With Lambdas C++14 Style and Dependently-Typed Curried printf. Both blogposts discuss currying of functions proper. I.e., they discuss how C++ can treat functions as values at runtime.

However, currying is not limited to just functions. Types can also be curried---if they take type arguments. In C++, we call them templates. Templates are "functions" at type level. For example, passing two type arguments std::string and int to std::map gives std::map<std::string, int>. So std::map is a type-level function that takes two (type) arguments and gives another type as a result. They are also known as type constructors.

So, the question today is: Can C++ templates be curried? As it turns out, they can be. Rather easily. So, here we go... #includ…

Non-colliding Efficient type_info::hash_code Across Shared Libraries

C++ standard library has std::type_info and std::type_index to get run-time type information about a type. There are some efficiency and robustness issues in using them (especially when dynamically loaded libraries are involved.)

TL;DR; The -D__GXX_MERGED_TYPEINFO_NAMES -rdynamic compiler/linker options (for both the main program and the library) generates code that uses pointer comparison in std::type_info::operator==().

The typeid keyword is used to obtain a type's run-time type information. Quoting cppreference. The typeid expression is an lvalue expression which refers to an object with static storage duration, of the polymorphic type const std::type_info or of some type derived from it.std::type_info objects can not be put in std::vector because they are non-copyable and non-assignable. Of course, you can have a std::vector<const std::type_info *> as the object returned by typeid has static storage duration. You could also use std::vector<std::type_index>. st…

Chained Functions Break Reference Lifetime Extension

I discovered a reference lifetime extension gotcha.

TL;DR; Chained functions (that return a reference to *this) do not trigger C++ reference lifetime extension. Four ways out: First, don't rely on lifetime extension---make a copy; Second, have all chained functions return *this by-value; Third, use rvalue reference qualified overloads and have only them return by-value; Fourth, have a last chained ExtendLifetime() function that returns a prvalue (of type *this). C++ Reference Lifetime Extension C++ has a feature called "reference lifetime extension". Consider the following. std::array<std::string, 5> create_array_of_strings(); { const std::array &arr = create_array_of_strings(); // Only inspect arr here. } // arr out of scope. The temporary pointed to by arr destroyed here. The temporary std::array returned by create_array_of_strings() is not destroyed after the function returns. Instead the "lifetime" of the temporary std::array is extended …

Convenient deduction guides for std::function

The objective is to allow the following to be valid C++. #include <functional> struct Test { void func(int) {} }; void test() { std::function deduced = &Test::func; // compiler error std::function<void (Test *, int)> ex = &Test::func; // OK } With class template argument deduction in C++17, type arguments for std::function above should have been deduced. The first line in function testfails to compile as of this writing because std::function does not appear to have deduction guides for conversion from pointer to member functions. On the other hand, explicitly specifying the template arguments makes the compiler happy.

The following deduction guide seems to fix the issue. namespace std { // warning: undefined behavior template<class R, class C, class... ArgTypes> function(R(C::*)(ArgTypes...)) -> function<R(C*, ArgTypes...)>; } void test() { std::function deduced = &Test::func; // Now, OK std::function<void (Test *, int)&…

Inheritance vs std::variant

C++17 added std::variant and std::visit in its repertoire. They are worth a close examination. I've been wondering about whether they are always better than inheritance for modeling sum-types (fancy name for discriminated unions) and if not, under what circumstances they are not. We'll compare the two approaches in this blog post. So here it goes.

Inheritancestd::variantNeed not know all the derived types upfront (open-world assumption)Must know all the cases upfront (closed-world assumption)Dynamic Allocation (usually)No dynamic allocationIntrusive (must inherit from the base class)Non-intrusive (third-party classes can participate)Reference semantics (think how you copy a vector of pointers to base class?)Value semantics (copying is trivial)Algorithm scattered into classesAlgorithm in one placeLanguage supported (Clear errors if pure-virtual is not implemented)Library supported (poor error messages)Creates a first-class abstractionIt’s just a containerKeeps fluent interfaces…