Shallow Copy vs. Deep Copy

This lesson covers the difference between a shallow copy and a deep copy in C, so you can avoid bugs caused by unintended data sharing. It's for anyone searching "C shallow copy deep copy."

Assigning a C struct directly copies it by value, but if the struct contains a pointer, the data the pointer points to isn't duplicated along with it. This is the state called a "shallow copy."

The example's Person struct has a pointer field, Address *address. Copy this struct directly, and both the copy and the original still point at the same Address — changing one affects the other.

A common early mistake is the bug of "I thought I made a copy, but the original data changed too." This happens especially often when handling a struct that contains a pointer.

In real projects, implementing a "deep copy" — newly allocating and copying the data a pointer points to as well — is what's needed to prevent unintended data sharing.

📖 Reference code

#include <stdio.h>
#include <string.h>

typedef struct {
    char city[20];
} Address;

typedef struct {
    char name[20];
    Address *address;
} Person;

int main(void) {
    Address addr;
    strcpy(addr.city, "Tokyo");
    Person original;
    strcpy(original.name, "Alice");
    original.address = &addr;

    Person shallow = original;
    strcpy(shallow.address->city, "Osaka");
    printf("%s\n", original.address->city);
    return 0;
}

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🧪 This site can't compile or run C directly, so it checks on the spot whether what you typed matches the reference code (scoring happens entirely in your browser — nothing is sent anywhere).