[PATCH] libstdc++: Fix-self element self-assigments when insertint an empty range [PR121313]

[Patrick Palka]

On Thu, 14 Aug 2025, Tomasz Kamiński wrote:

Typo in subject line, "inserting"

> For __n == 0, the elements where self move-assigned by
> std::move_backward(__ins, __old_finish - __n, __old_finish).

s/where/were?

LGTM besides that.

> 
> 	PR libstdc++/121313
> 
> libstdc++-v3/ChangeLog:
> 
> 	* include/bits/vector.tcc (vector::insert_range): Add check for
> 	empty size.
> 	* testsuite/23_containers/vector/modifiers/insert/insert_range.cc:
> 	New tests.
> ---
> Testing on x86_64-linux. Newly added test passed.
> 
> OK for trunk and v15.
> 
>  libstdc++-v3/include/bits/vector.tcc          | 10 ++--
>  .../vector/modifiers/insert/insert_range.cc   | 50 +++++++++++++++++++
>  2 files changed, 57 insertions(+), 3 deletions(-)
> 
> diff --git a/libstdc++-v3/include/bits/vector.tcc b/libstdc++-v3/include/bits/vector.tcc
> index 70ead1d7083..1543ce9cd77 100644
> --- a/libstdc++-v3/include/bits/vector.tcc
> +++ b/libstdc++-v3/include/bits/vector.tcc
> @@ -1007,17 +1007,21 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
>  
>  	if constexpr (ranges::forward_range<_Rg>)
>  	  {
> +	    const auto __ins_idx = __pos - cbegin();
> +	    // Number of new elements to insert:
> +	    const auto __n = size_type(ranges::distance(__rg));
> +	    if (__n == 0)
> +	      return begin() + __ins_idx;
> +
>  	    // Start of existing elements:
>  	    pointer __old_start = this->_M_impl._M_start;
>  	    // End of existing elements:
>  	    pointer __old_finish = this->_M_impl._M_finish;
>  	    // Insertion point:
> -	    const auto __ins_idx = __pos - cbegin();
>  	    pointer __ins = __old_start + __ins_idx;
> -	    // Number of new elements to insert:
> -	    const auto __n = size_type(ranges::distance(__rg));
>  	    // Number of elements that can fit in unused capacity:
>  	    const auto __cap = this->_M_impl._M_end_of_storage - __old_finish;
> +
>  	    if (__cap >= __n)
>  	      {
>  		// Number of existing elements after insertion point:
> diff --git a/libstdc++-v3/testsuite/23_containers/vector/modifiers/insert/insert_range.cc b/libstdc++-v3/testsuite/23_containers/vector/modifiers/insert/insert_range.cc
> index 506bebbe519..e4b5982188f 100644
> --- a/libstdc++-v3/testsuite/23_containers/vector/modifiers/insert/insert_range.cc
> +++ b/libstdc++-v3/testsuite/23_containers/vector/modifiers/insert/insert_range.cc
> @@ -99,8 +99,58 @@ test_ranges()
>    return true;
>  }
>  
> +struct SelfAssignChecker {
> +  static int moveCounter;
> +  static int copyCounter;
> +
> +  SelfAssignChecker() = default;
> +  constexpr SelfAssignChecker(int v) : val(v) { }
> +  SelfAssignChecker(const SelfAssignChecker&) = default;
> +  SelfAssignChecker(SelfAssignChecker&&) = default;
> +
> +  SelfAssignChecker operator=(const SelfAssignChecker& rhs)
> +  {
> +    if (this == &rhs)
> +      ++copyCounter;
> +    this->val = rhs.val;
> +    return *this;
> +  }
> +
> +  SelfAssignChecker operator=(SelfAssignChecker&& rhs)
> +  {
> +    if (this == &rhs)
> +      ++moveCounter;
> +    this->val = rhs.val;
> +    return *this;
> +  }
> +
> +  int val;
> +
> +  friend bool operator==(SelfAssignChecker, SelfAssignChecker) = default;
> +};
> +
> +int SelfAssignChecker::moveCounter = 0;
> +int SelfAssignChecker::copyCounter = 0;
> +
> +void
> +test_pr121313()
> +{
> +  using namespace __gnu_test;
> +
> +  SelfAssignChecker::copyCounter = SelfAssignChecker::moveCounter = 0;
> +  do_test<test_forward_range<int>, std::allocator<SelfAssignChecker>>();
> +  VERIFY( SelfAssignChecker::moveCounter == 0 );
> +  VERIFY( SelfAssignChecker::copyCounter == 0 );
> +
> +  SelfAssignChecker::copyCounter = SelfAssignChecker::moveCounter = 0;
> +  do_test<test_input_range<int>, std::allocator<SelfAssignChecker>>();
> +  VERIFY( SelfAssignChecker::moveCounter == 0 );
> +  VERIFY( SelfAssignChecker::copyCounter == 0 );
> +}
> +
>  int main()
>  {
>    test_ranges();
> +  test_pr121313();
>    static_assert( test_ranges() );
>  }
> -- 
> 2.49.0
> 
>