# Overflow of C++ containers Detects when you access a C++ container outside its bounds. ## Overview Use this check to detect when you access a libc++ container beyond the region `[container.begin(), container.end()]`, even when the accessed memory is in a heap-allocated buffer the container uses internally. Available in Xcode 7 and later. ### Vector overflow in C++ In the following example, the `vector` variable has valid indexes in the range `[0,2]`, but the accessed index is `3`, which causes an overflow: ```occ std::vector vector; vector.push_back(0); vector.push_back(1); vector.push_back(2); auto *pointer = &vector[0]; return pointer[3]; // Error: out of bounds access for vector ``` #### Solution Add a bounds check before attempting to access a container at a specific index. ### Disabling Container Overflow Checks You may encounter a false-positive ‘Container overflow’ error when code that isn’t compiled with Address Sanitizer modifies a container. For container overflow checks to work correctly, you need to compile all code with Address Sanitizer. If you can’t do this, turn off container overflow checks using one of the following methods: Set the [Enable C++ Container Overflow Checks](/documentation/Xcode/build-settings-reference#Enable-C++-Container-Overflow-Checks) build setting to ‘NO’. - Set the ASAN_OPTIONS Environment Variable: Set the `ASAN_OPTIONS` environment variable to `detect_container_overflow=0`, or append `:detect_container_overflow=0` to this environment variable if it has already been set. You should do this under the Scheme for Run targets, or Configurations for Test Plans. Note that for UI tests you may need to set this in the XCUIApplication . - Define the __asan_default_options function in your executable: Use this method when you can’t control your program’s environment variables. Disable container overflow checks by defining the following function in your executable: ```occ #ifdef __cplusplus extern "C" { #endif #include __attribute__((used, visibility("default"))) const char *__asan_default_options() { return "detect_container_overflow=0"; } #ifdef __cplusplus } #endif ``` If you set the [Exported Symbols File](/documentation/Xcode/build-settings-reference#Exported-Symbols-File) build setting, then also add `___asan_default_options` to the file to ensure that the system exports the symbol. If you set the `detect_container_overflow` option in both the `__asan_default_options` function, and the `ASAN_OPTIONS` environment variable, the system uses the value in the environment variable. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)