lambda returns incorrect values when variable is initialised inside lambda but not if initialised in outer scope

Question

Given this code example:

#include <vector>
#include <cstdint>
#include <memory>
#include <iostream>
#include <functional>

enum StudentType { BURSARY, PAYING };

class Student {
public:
    Student(const std::vector<int>& grades, StudentType type) : m_grades(grades), m_type(type) {}

    uint8_t gradeByIndex(size_t index) { return m_grades[index]; }
    StudentType getType() { return m_type; }
    size_t numGrades() { return m_grades.size(); }

private:
    std::vector<int> m_grades;
    StudentType m_type;
};


int main() {

    std::vector<int> grades1{ 1,2,3 };
    std::vector<int> grades2{ 4,5,6 };
    std::vector<int> grades3{ 7,8,9 };
    std::vector<int> grades4{ 10,11,12 };
    std::vector<int> grades5{ 13,14,15 };

    using func = std::function<std::unique_ptr<Student>()>;

    std::vector<func> funcs;

    using StudentGradesAndType = std::pair<std::vector<int>, StudentType>;

    const StudentGradesAndType StudentList[]{ {grades1, BURSARY},
                                 {grades1, BURSARY},
                                 {grades1, PAYING},
                                 {grades1, PAYING},
                                 {grades1, PAYING} };


    for (auto& entry : StudentList)
    {
        Student student(entry.first, entry.second);
        auto mylambda = [&student]() {
            return std::make_unique<Student>(student);
        };

        funcs.push_back(mylambda);
    }

    for (auto el : funcs) {
        std::cout << el()->numGrades() << std::endl;
    }
}

With the code above, the output is zero for each student.

However, if I change the code in the for loop like this:

for (auto& entry : StudentList)
{
    auto mylambda = [&entry]() {
        Student student(entry.first, entry.second);
        return std::make_unique<Student>(student);
    };
    funcs.push_back(mylambda);
}

Then the output is 3 for each student as expected.

Why does the first code version not work?

Alan Birtles · Accepted Answer · 2021-04-11 17:41:59Z

2

The captured student variable is a reference to a local variable therefore your lambda has undefined behaviour if called after student goes out of scope. Capture by value to prevent this problem.

The captured entry is a reference to an element in StudentList so the lambda works correctly as long as the reference is still valid within StudentList.

answered Apr 11, 2021 at 17:41

Alan Birtles

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