revision #2: 检阅部分模板相关内容

code/2/2.12.external.template.cpp

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+//
+// 2.12.external.template.cpp
+// chapter 2 language usability
+// modern cpp tutorial
+//
+// created by changkun at changkun.de
+//
+
+#include <iostream>
+#include <vector>
+
+template class std::vector<bool>;           // forcely instantiation
+extern template class std::vector<double>;  // external template for avoiding instantiation in this file
+
+template<bool T> class MagicType {
+    bool magic = T;
+};
+
+int main() {
+    // the >> in template
+    std::vector<std::vector<int>> matrix;
+    std::vector<MagicType<(1>2)>> magic;
+}