[core] Return {Source,Layer}::getID by value

Avoid dangling references in the following sequence:

    auto& id = layer->getID();
    layer->setMaxZoom(2);
    std::cout << id;

The reference would be dangling because mutating the layer allocates a new Immutable impl, and there may be no references to the prior impl, which held the id.

2 files changed, 2 insertions, 2 deletions

diff --git a/src/mbgl/style/layer.cpp b/src/mbgl/style/layer.cpp
index fe51dd09ca..142fe313cf 100644
--- a/src/mbgl/style/layer.cpp
+++ b/src/mbgl/style/layer.cpp
@@ -18,7 +18,7 @@ LayerType Layer::getType() const {
     return baseImpl->type;
 }
 
-const std::string& Layer::getID() const {
+std::string Layer::getID() const {
     return baseImpl->id;
 }
 
diff --git a/src/mbgl/style/source.cpp b/src/mbgl/style/source.cpp
index 4fcebc8377..e7701b8bec 100644
--- a/src/mbgl/style/source.cpp
+++ b/src/mbgl/style/source.cpp
@@ -19,7 +19,7 @@ SourceType Source::getType() const {
     return baseImpl->type;
 }
 
-const std::string& Source::getID() const {
+std::string Source::getID() const {
     return baseImpl->id;
 }