c++ lambda executed at wrong address / in wrong context - c++

Question

The following code is also available at godbolt.org/z/SjkAWZ.
I have a struct with a std::function as member variable, which gets assigned by a lambda expression in the constructor.
The lambda can be called from the constructor successfully, but not from a member function.
#include <vector>
#include <functional>
struct S
{
S()
{
std::cout << "construct: " << reinterpret_cast<long>(this) << " " << std::flush;
std::cout << m_ << '\n';
lambda_ = [&]() {
std::cout << "lambda: " << reinterpret_cast<long>(this) << " " << std::flush;
std::cout << m_ << '\n';
};
lambda_();
}
void foo()
{
std::cout << "foo: " << reinterpret_cast<long>(this) << " " << std::flush;
std::cout << m_ << '\n';
lambda_();
}
private:
int m_ = 3; // some member variable
std::function<void(void)> lambda_;
};
int main()
{
std::vector<S> v;
v.emplace_back();
v.emplace_back();
std::cout << "\n--- loop ---\n";
for(auto& b : v)
{ b.foo(); }
}
Possible program output:
construct: 94159000759920 3
lambda: 94159000759920 3
construct: 94159000761048 3
lambda: 94159000761048 3
--- loop ---
foo: 94159000761008 3
lambda: 94159000759920 0 // <-- bad *this* address, bad value for m_
foo: 94159000761048 3
lambda: 94159000761048 3 // <-- bad *this* address, correct value for m_ (coincidence?)
As you can see, the context this of the lambda function is wrong if it gets called from the member function foo.
Interestingly, this is not the case if I only emplace one element in the vector:
construct: 13753968 3
lambda: 13753968 3
--- loop ---
foo: 13753968 3
lambda: 13753968 3 // <-- correct *this* address and correct value for m_
I am using g++ version 8.3.0 on Debian.
Am I causing undefined behaviour by constructing the elements in the vector in place?

Answer 1

You missed the copy / move constructor for your struct!
If you assign some vars to a std::vector it will reserve sometimes new memory and copy all content to the new memory. In your case, the lambda is not aware about the change and keeps the reference to the old instance of S. So your code is simply broken.
You can work around the problem by reserving enough space before you assign content with:
int main()
{
std::vector<S> v;
v.reserve(5);
...
}
But this is only a work around! You should prepare a move/copy constructor to be able to get correct copies of your instances.