template_on_header

Author: cirosantilli

cpp/template_on_header/Makefile

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+../../Makefile_one

cpp/template_on_header/Makefile_params

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+CXX_STD = c++11

cpp/template_on_header/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_on_header/main.cpp

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+#include <iostream>
+
+#include "mytemplate.hpp"
+#include "notmain.hpp"
+
+int main() {
+    std::cout << notmain() + MyTemplate<int>().f(1) << std::endl;
+}

cpp/template_on_header/mytemplate.hpp

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+#ifndef MYTEMPLATE_HPP
+#define MYTEMPLATE_HPP
+
+template<class T>
+struct MyTemplate {
+    T f(T t) { return t + 1; }
+};
+
+#endif

cpp/template_on_header/notmain.cpp

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+#include "mytemplate.hpp"
+#include "notmain.hpp"
+
+int notmain() { return MyTemplate<int>().f(1); }

cpp/template_on_header/notmain.hpp

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+#ifndef NOTMAIN_HPP
+#define NOTMAIN_HPP
+
+int notmain();
+
+#endif