Fix places that trigger no-unused-lambda-capture

webrtc/modules/audio_processing/aec3/adaptive_fir_filter.cc

 /*
  *  Copyright (c) 2017 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 489 matching lines @@
       aec3::UpdateErlEstimator(H2_, &erl_);
   }
 }
 
 // Constrains the a partiton of the frequency domain filter to be limited in
 // time via setting the relevant time-domain coefficients to zero.
 void AdaptiveFirFilter::Constrain() {
   std::array<float, kFftLength> h;
   fft_.Ifft(H_[partition_to_constrain_], &h);
 
-  constexpr float kScale = 1.0f / kFftLengthBy2;
+  const float kScale = 1.0f / kFftLengthBy2;

[nisse-webrtc, 2017/08/23 09:42:02]

Could add a comment here too, on why constexpr doesn't work.

[eladalon, 2017/08/23 11:42:53]

IMHO, it would be overkill to fill the code with too many comments about this.
If someone ever tries to "fix" this, the trybots would prevent that. Or, if it
ever starts passing, and someone fixes it then - great.

   std::for_each(h.begin(), h.begin() + kFftLengthBy2,
                 [kScale](float& a) { a *= kScale; });

[kwiberg-webrtc, 2017/08/23 11:35:59]

Does it work if you change the capture list to [&] instead? That's allowed since
the lambda doesn't outlive the scope in which it is created.

[eladalon, 2017/08/23 11:42:53]

You can't capture a constexpr by reference, can you?

[kwiberg-webrtc, 2017/08/23 11:56:40]

Yes, you can. Unlike preprocessor constants and enum values, they are lvalues,
so you can take their address etc.

[eladalon, 2017/08/23 12:17:30]

Surprising, but OK. Nevertheless, I am not sure that would be an improvement of
the code. I think it would be better with const than with an unlimited capture.
Unless you feel strongly about this, I'd rather not make that change.

   std::fill(h.begin() + kFftLengthBy2, h.end(), 0.f);
 
   std::copy(h.begin(), h.begin() + kFftLengthBy2,
             h_.begin() + partition_to_constrain_ * kFftLengthBy2);
 
   fft_.Fft(&h, &H_[partition_to_constrain_]);
 
   partition_to_constrain_ = partition_to_constrain_ < (H_.size() - 1)
                                 ? partition_to_constrain_ + 1
                                 : 0;
 }
 
 }  // namespace webrtc