C++ linker presentation

cpplinker/07_linker_errors/multiple_def/a.cpp

@@ -0,0 +1,6 @@
+#include <iostream>
+#include "shared.h"
+
+void print_a() {
+    std::cout << "a: globalVar = " << globalVar << "\n";
+}

cpplinker/07_linker_errors/multiple_def/b.cpp

@@ -0,0 +1,6 @@
+#include <iostream>
+#include "shared.h"
+
+void print_b() {
+    std::cout << "b: globalVar = " << globalVar << "\n";
+}

cpplinker/07_linker_errors/multiple_def/build.sh

@@ -0,0 +1,10 @@
+#!/bin/bash
+
+echo "=== This build SHOULD fail with 'multiple definition' ==="
+echo ""
+g++ -std=c++17 a.cpp b.cpp main.cpp -o app 2>&1 || true
+echo ""
+echo "Both a.o and b.o define globalVar because shared.h has a definition."
+echo ""
+echo "Fix 1: use 'extern int globalVar;' in header, define in one .cpp"
+echo "Fix 2: use 'inline int globalVar = 42;' (C++17)"

cpplinker/07_linker_errors/multiple_def/main.cpp

@@ -0,0 +1,8 @@
+void print_a();
+void print_b();
+
+int main() {
+    print_a();
+    print_b();
+    return 0;
+}

cpplinker/07_linker_errors/multiple_def/shared.h

@@ -0,0 +1,13 @@
+#pragma once
+
+// BUG: this is a definition, not just a declaration.
+// Including this header in multiple TUs causes "multiple definition" errors.
+int globalVar = 42;
+
+// FIX (pick one):
+// 1. Use extern in header + define in one .cpp:
+//    extern int globalVar;          // in header
+//    int globalVar = 42;            // in one .cpp
+//
+// 2. Use inline (C++17):
+//    inline int globalVar = 42;