[libcamera-devel] [PATCH v2] libcamera: pipeline: uvcvideo: Avoid reference to temporary object
Laurent Pinchart
laurent.pinchart at ideasonboard.com
Sun Mar 28 04:57:41 CEST 2021
From: Khem Raj <raj.khem at gmail.com>
A range-based for loop whose range expression is an array of char
pointers and range variable declaration is a const reference to a
std::string creates a temporary string from the char pointer and binds
the range variable reference to it. This creates a const reference to a
temporary, which is valid in C++, and extends the lifetime of the
temporary to the lifetime of the reference.
However, lifetime extension in range-based for loops is considered as a
sign of a potential issue, as a temporary is created for every
iteration, which can be costly, and the usage of a reference in the
range declaration doesn't make it obvious that the code isn't simply
binding a reference to an existing object. gcc 11, with the
-Wrange-loop-construct option, flags this:
uvcvideo.cpp:432:33: error: loop variable 'name' of type 'const string&' {aka 'const std::__cxx11::basic_string<cha
r>&'} binds to a temporary constructed from type 'const char* const' [-Werror=range-loop-construct]
| 432 | for (const std::string &name : { "idVendor", "idProduct" }) {
| | ^~~~
To please the compiler, make the range variable a const char *. This may
bring a tiny performance improvement, as the name is only used once, in
a location where the compiler can use
operator+(const std::string &, const char *)
instead of
operator+(const std::string &, const std::string &)
Signed-off-by: Khem Raj <raj.khem at gmail.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
Use a const char * type instead of auto, and update the commit message
accordingly.
Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
---
src/libcamera/pipeline/uvcvideo/uvcvideo.cpp | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/src/libcamera/pipeline/uvcvideo/uvcvideo.cpp b/src/libcamera/pipeline/uvcvideo/uvcvideo.cpp
index 031f96e28449..b6c6ade5ebaf 100644
--- a/src/libcamera/pipeline/uvcvideo/uvcvideo.cpp
+++ b/src/libcamera/pipeline/uvcvideo/uvcvideo.cpp
@@ -429,7 +429,7 @@ std::string PipelineHandlerUVC::generateId(const UVCCameraData *data)
/* Creata a device ID from the USB devices vendor and product ID. */
std::string deviceId;
- for (const std::string &name : { "idVendor", "idProduct" }) {
+ for (const char *name : { "idVendor", "idProduct" }) {
std::ifstream file(path + "/../" + name);
if (!file.is_open())
--
Regards,
Laurent Pinchart
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