C++ code crashing when using function instead of inline code

Question

C++ rookie here.

I have the following code:

std::vector<float> MyBuffer::readAverage(int numberOfBuffers) {
 std::vector<float> result = std::vector<float>(streams.size());
 for (int i = 0; i < streams.size(); ++i) {
    result[i] = getAverage(streams[i], numberOfBuffers);
 }
 return result;
}

float MyBuffer::getAverage(std::deque<float> input, int numberOfBuffers) {
 float sum = 0;
 for (int i = 0; i < numberOfBuffers; ++i) {
     sum += input[i];
 }
 return sum / numberOfBuffers;
}

This code randomly crashes at getAverage(), I am not sure why.

Strange thing (for me as a C++ rookie at least) is that when I inline the function, it does not crash:

std::vector<float> MyBuffer::readAverage(int numberOfBuffers) {
 std::vector<float> result = std::vector<float>(streams.size());
 for (int i = 0; i < streams.size(); ++i) {
    float sum = 0;
    for (int i1 = 0; i1 < numberOfBuffers; ++i1) {
        sum += streams[i][i1];
    }
    result[i] = sum / numberOfBuffers;
 }
 return result;
}

I can understand that there may be many reasons why this specific code is crashing - so my question relates more to what changes when I inline it, rather than calling a function? In my mind it should be exactly the same thing, but I guess there is something about the way C++ works that I am not grasping?

Probably division by zero (check numberOfBuffers) or out of bounds access for input. You can replace sum += input[i]; with sum += input.at(i); to enforce bounds checking. — Rost
– Rost, Commented Aug 30, 2019 at 17:13
getAverage makes a copy of std::deque<float> and that could run out of memory. — Maxim Egorushkin
– Maxim Egorushkin, Commented Aug 30, 2019 at 17:15
In my mind it should be exactly the same thing. No, by passing it into a function by value you are creating a copy. Try passing it as const deque<float>& input instead. — ph3rin
– ph3rin, Commented Aug 30, 2019 at 17:15
std::vector<float> result = std::vector<float>(streams.size()); Why copy-initialisation? Just construct the object normally: std::vector<float> result(streams.size()); — Lightness Races in Orbit
– Lightness Races in Orbit, Commented Aug 30, 2019 at 17:19

Hemantharaju .H · Accepted Answer · 2019-08-30 18:58:43Z

1

The program has many potential reasons why it can cause a crash.
1. bufferDurationMs is not initialized in the provided code, I hope its initialized to value other than 0.
2. for (int i = 0; i < streams.size(); ++i) { result[i] = getAverage(streams[i], numberOfBuffers); } use result.size() instead of streams.size() as result is lvalue. It is better to check both of these conditions in for.
3. It is quite possible that numberOfBuffers can be 0 in which case code would crash(divide by zero)
Some optimizations that can be done in the code:
1. std::vector<float> result = std::vector<float>(streams.size()); use reserve rather than using a costly operation of creating a vector and assigning it to lvalue.
  std::vector result; result.reserve(streams.size());
2. float MyBuffer::getAverage(std::deque<float> input, int numberOfBuffers) prefer const reference rather than creating a copy of an object
  
  const std::deque& input

answered Aug 30, 2019 at 18:58

Hemantharaju .H

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1 Comment

HPage Over a year ago

Seems like the main issue was the fact that I was creating a copy of input. Passing const std::deque<float>& input solved it.

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