template_instantiation

Author: cirosantilli

cpp/README.md

@@ -66,6 +66,8 @@
     1.  [Preprocessor](preprocessor.cpp)
     1.  using
         1. using type alias. TODO http://en.cppreference.com/w/cpp/language/type_alias
+    1.  templates
+        1.  [template instantiation](template_instantiation/)
 1.  [Standard library](standard_library.md)
     1.  [Headers](common.hpp)
     1.  [Containers](containers.md)

cpp/template_instantiation/Makefile

@@ -0,0 +1 @@
+../../Makefile_one

cpp/template_instantiation/Makefile_params

@@ -0,0 +1 @@
+CXX_STD = c++11

cpp/template_instantiation/README.md

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+Template instantiation saves us from infinite compilation times because whenever you modify a header, the build system (correctly) rebuilds everything that includes that header.
+
+They can also save object size (and likely a little bit of time) because templates are re-instantiated for each object file, unless users mark it as extern on every file, which they won't remember to do
+
+* https://stackoverflow.com/questions/8130602/using-extern-template-c11/59614090#59614090
+* https://stackoverflow.com/questions/2351148/explicit-instantiation-when-is-it-used
+* https://en.cppreference.com/w/cpp/language/class_template#Explicit_instantiation
+
+We can also confirm with `nm -C` that the `main.o` object file does not contain a definition of `MyClass`, in that case because `MyClass` is not a complete definition because `f` is not defined.

cpp/template_instantiation/main.cpp

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+#include <iostream>
+
+#include "notmain.hpp"
+
+int main() {
+    std::cout << MyClass<int>().f(1) << std::endl;
+#if 0
+    // error: undefined reference to `X<float>::f(float)'
+    std::cout << X<float>().f(1.5) << std::endl;
+#endif
+}

cpp/template_instantiation/notmain.cpp

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+#include "notmain.hpp"
+
+template<class T>
+T MyClass<T>::f(T t) { return t + 1; }
+
+// Only int is instantiated.
+template class MyClass<int>;

cpp/template_instantiation/notmain.hpp

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+#ifndef NOTMAIN_HPP
+#define NOTMAIN_HPP
+
+template<class T> struct MyClass {
+    T f(T t);
+};
+
+#endif