Pointers

Null Pointer

nullptr means a pointer is not pointing at an object.

nullptr Check a pointer before dereferencing when it might be `nullptr`.

Null Pointer

example
null_pointer.cpp
Replay: real traced execution (multi-file project)
#include <iostream>

int main() {
    bool hasValue = false;
    int value = 42;
    int fallback = -1;
    int* maybeValue = nullptr;

    if (hasValue) {
        maybeValue = &value;
    }

    int result = fallback;
    if (maybeValue != nullptr) {
        result = *maybeValue;
    }

    std::cout << "hasValue=" << hasValue << std::endl;
    std::cout << "result=" << result << std::endl;
    return 0;
}

#include <iostream>

int main() {
    bool hasValue = true;
    int value = 42;
    int fallback = -1;
    int* maybeValue = nullptr;

    if (hasValue) {
        maybeValue = &value;
    }

    int result = fallback;
    if (maybeValue != nullptr) {
        result = *maybeValue;
    }

    std::cout << "hasValue=" << hasValue << std::endl;
    std::cout << "result=" << result << std::endl;
    return 0;
}

#include <iostream>

int main() {
    bool hasValue = false;
    int value = 7;
    int fallback = -1;
    int* maybeValue = nullptr;

    if (hasValue) {
        maybeValue = &value;
    }

    int result = fallback;
    if (maybeValue != nullptr) {
        result = *maybeValue;
    }

    std::cout << "hasValue=" << hasValue << std::endl;
    std::cout << "result=" << result << std::endl;
    return 0;
}

#include <iostream>

int main() {
    bool hasValue = false;
    int value = 99;
    int fallback = -1;
    int* maybeValue = nullptr;

    if (hasValue) {
        maybeValue = &value;
    }

    int result = fallback;
    if (maybeValue != nullptr) {
        result = *maybeValue;
    }

    std::cout << "hasValue=" << hasValue << std::endl;
    std::cout << "result=" << result << std::endl;
    return 0;
}

  1. hasValue ← 0, value ← 42, fallback ← -1, maybeValue ← 0, result ← -1

    3int main() {4    bool hasValue→ 0 = false; //@hasValue=true5    int value→ 42 = 42; //@value=7, 996    int fallback→ -1 = -1;7    int* maybeValue→ 0 = nullptr;89    if (hasValue) {10        maybeValue = &value;11    }1213    int result→ -1 = fallback-1;14    if (maybeValue != nullptr) {15        result = *maybeValue;16    }1718    std::cout << "hasValue=" << hasValue0 << std::endl;19    std::cout << "result=" << result-1 << std::endl;20    return 0;21}
    outputhasValue=0
result=-1

  1. hasValue ← 1, value ← 42, fallback ← -1, maybeValue ← 0

    3int main() {4    bool hasValue→ 1 = true;5    int value→ 42 = 42;6    int fallback→ -1 = -1;7    int* maybeValue→ 0 = nullptr;

  2. maybeValue ← ⟨addr A⟩

    9if (hasValue1) {10    maybeValue→ ⟨addr A⟩ = &value42;11}

  3. result ← -1

    13int result→ -1 = fallback-1;14if (maybeValue != nullptr) {

  4. result ← 42

    13int result = fallback;14if (maybeValue⟨addr A⟩ != nullptr) {15    result→ 42 = *maybeValue⟨addr A⟩;16}

  5. std::cout << "hasValue=" << hasValue << std::endl;

    18    std::cout << "hasValue=" << hasValue1 << std::endl;19    std::cout << "result=" << result42 << std::endl;20    return 0;21}
    outputhasValue=1
result=42

  1. hasValue ← 0, value ← 7, fallback ← -1, maybeValue ← 0, result ← -1

    3int main() {4    bool hasValue→ 0 = false;5    int value→ 7 = 7;6    int fallback→ -1 = -1;7    int* maybeValue→ 0 = nullptr;89    if (hasValue) {10        maybeValue = &value;11    }1213    int result→ -1 = fallback-1;14    if (maybeValue != nullptr) {15        result = *maybeValue;16    }1718    std::cout << "hasValue=" << hasValue0 << std::endl;19    std::cout << "result=" << result-1 << std::endl;20    return 0;21}
    outputhasValue=0
result=-1

  1. hasValue ← 0, value ← 99, fallback ← -1, maybeValue ← 0, result ← -1

    3int main() {4    bool hasValue→ 0 = false;5    int value→ 99 = 99;6    int fallback→ -1 = -1;7    int* maybeValue→ 0 = nullptr;89    if (hasValue) {10        maybeValue = &value;11    }1213    int result→ -1 = fallback-1;14    if (maybeValue != nullptr) {15        result = *maybeValue;16    }1718    std::cout << "hasValue=" << hasValue0 << std::endl;19    std::cout << "result=" << result-1 << std::endl;20    return 0;21}
    outputhasValue=0
result=-1