[C++] Fix warnings. #979

Job	Run time
C# 3.1.301	2m 28s
C++ GCC 6 (Ubuntu 20.04)	1m 15s
Java 8 (ubuntu-22.04)	1m 16s
C++ GCC 11 (Ubuntu 22.04)	1m 24s
C++ GCC 7 (Ubuntu 20.04)	1m 25s
Java 8 (windows-latest)	3m 23s
Java 17 (ubuntu-22.04)	1m 29s
C++ GCC 8 (Ubuntu 20.04)	1m 27s
C++ GCC 12 (Ubuntu 22.04)	1m 29s
Java 17 (windows-latest)	4m 12s
C++ GCC 9 (Ubuntu 20.04)	1m 29s
C++ GCC 13 (Ubuntu 22.04)	1m 23s
Java 21 (ubuntu-22.04)	1m 29s
C++ GCC 10 (Ubuntu 20.04)	1m 27s
Java 21 (windows-latest)	2m 25s
C++ Clang 9 (Ubuntu 20.04)	1m 15s
C++ Clang 13 (Ubuntu 22.04)	1m 8s
C++ Xcode (macOS)	2m 0s
C++ Clang 14 (Ubuntu 22.04)	1m 16s
C++ Clang 15 (Ubuntu 22.04)	1m 12s
C++ Clang 10 (Ubuntu 20.04)	1m 23s
C++ Clang 16 (Ubuntu 22.04)	1m 15s
C++ MSVC (Windows) (windows-latest)	3m 7s
C++ Clang 11 (Ubuntu 20.04)	1m 29s
C++ Clang 17 (Ubuntu 22.04)	1m 18s
C++ Clang 12 (Ubuntu 20.04)	1m 31s
Rust stable	40s
Rust beta	51s
Rust nightly	47s
	47m 13s

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