diff --git a/tests/unit/libstore/nix_api_store.cc b/tests/unit/libstore/nix_api_store.cc
index a31d66a4c..7c6ec0780 100644
--- a/tests/unit/libstore/nix_api_store.cc
+++ b/tests/unit/libstore/nix_api_store.cc
@@ -4,14 +4,10 @@
 #include "nix_api_store_internal.h"
 
 #include "tests/nix_api_store.hh"
+#include "tests/string_callback.hh"
 
 namespace nixC {
 
-void observe_string_cb(const char * start, unsigned int n, std::string * user_data)
-{
-    *user_data = std::string(start);
-}
-
 std::string PATH_SUFFIX = "/g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-name";
 
 TEST_F(nix_api_util_context, nix_libstore_init)
@@ -23,7 +19,7 @@ TEST_F(nix_api_util_context, nix_libstore_init)
 TEST_F(nix_api_store_test, nix_store_get_uri)
 {
     std::string str;
-    auto ret = nix_store_get_uri(ctx, store, (void *) observe_string_cb, &str);
+    auto ret = nix_store_get_uri(ctx, store, OBSERVE_STRING(str));
     ASSERT_EQ(NIX_OK, ret);
     ASSERT_STREQ("local", str.c_str());
 }
@@ -56,7 +52,7 @@ TEST_F(nix_api_store_test, DoesNotCrashWhenContextIsNull)
 TEST_F(nix_api_store_test, get_version)
 {
     std::string str;
-    auto ret = nix_store_get_version(ctx, store, (void *) observe_string_cb, &str);
+    auto ret = nix_store_get_version(ctx, store, OBSERVE_STRING(str));
     ASSERT_EQ(NIX_OK, ret);
     ASSERT_STREQ(PACKAGE_VERSION, str.c_str());
 }
@@ -69,7 +65,7 @@ TEST_F(nix_api_util_context, nix_store_open_dummy)
     ASSERT_STREQ("dummy", store->ptr->getUri().c_str());
 
     std::string str;
-    nix_store_get_version(ctx, store, (void *) observe_string_cb, &str);
+    nix_store_get_version(ctx, store, OBSERVE_STRING(str));
     ASSERT_STREQ("", str.c_str());
 
     nix_store_free(store);
diff --git a/tests/unit/libutil-support/tests/string_callback.cc b/tests/unit/libutil-support/tests/string_callback.cc
new file mode 100644
index 000000000..28ac8b10c
--- /dev/null
+++ b/tests/unit/libutil-support/tests/string_callback.cc
@@ -0,0 +1,9 @@
+#include "string_callback.hh"
+
+namespace nix::testing {
+
+void observe_string_cb(const char * start, unsigned int n, std::string * user_data) {
+    *user_data = std::string(start);
+}
+
+}
diff --git a/tests/unit/libutil-support/tests/string_callback.hh b/tests/unit/libutil-support/tests/string_callback.hh
new file mode 100644
index 000000000..808fb707b
--- /dev/null
+++ b/tests/unit/libutil-support/tests/string_callback.hh
@@ -0,0 +1,12 @@
+#pragma once
+#include <string>
+
+namespace nix::testing {
+
+void observe_string_cb(const char * start, unsigned int n, std::string * user_data);
+inline void * observe_string_cb_data(std::string & out) {
+    return (void *) &out;
+};
+#define OBSERVE_STRING(str) (void *)nix::testing::observe_string_cb, nix::testing::observe_string_cb_data(str)
+
+}
diff --git a/tests/unit/libutil/nix_api_util.cc b/tests/unit/libutil/nix_api_util.cc
index 09f3f3e05..d2999f55b 100644
--- a/tests/unit/libutil/nix_api_util.cc
+++ b/tests/unit/libutil/nix_api_util.cc
@@ -3,16 +3,12 @@
 #include "nix_api_util.h"
 #include "nix_api_util_internal.h"
 #include "tests/nix_api_util.hh"
+#include "tests/string_callback.hh"
 
 #include <gtest/gtest.h>
 
 namespace nixC {
 
-void observe_string_cb(const char * start, unsigned int n, std::string * user_data)
-{
-    *user_data = std::string(start);
-}
-
 TEST_F(nix_api_util_context, nix_context_error)
 {
     std::string err_msg_ref;
@@ -62,10 +58,10 @@ TEST_F(nix_api_util_context, nix_setting_get)
 {
     ASSERT_EQ(ctx->last_err_code, NIX_OK);
     std::string setting_value;
-    nix_err result = nix_setting_get(ctx, "invalid-key", (void *) observe_string_cb, &setting_value);
+    nix_err result = nix_setting_get(ctx, "invalid-key", OBSERVE_STRING(setting_value));
     ASSERT_EQ(result, NIX_ERR_KEY);
 
-    result = nix_setting_get(ctx, "setting-name", (void *) observe_string_cb, &setting_value);
+    result = nix_setting_get(ctx, "setting-name", OBSERVE_STRING(setting_value));
     ASSERT_EQ(result, NIX_OK);
     ASSERT_STREQ("empty", setting_value.c_str());
 }
@@ -79,7 +75,7 @@ TEST_F(nix_api_util_context, nix_setting_set)
     ASSERT_EQ(result, NIX_OK);
 
     std::string setting_value;
-    result = nix_setting_get(ctx, "setting-name", (void *) observe_string_cb, &setting_value);
+    result = nix_setting_get(ctx, "setting-name", OBSERVE_STRING(setting_value));
     ASSERT_EQ(result, NIX_OK);
     ASSERT_STREQ("new-value", setting_value.c_str());
 }
@@ -107,14 +103,14 @@ TEST_F(nix_api_util_context, nix_err_info_msg)
     std::string err_info;
 
     // no error
-    EXPECT_THROW(nix_err_info_msg(NULL, ctx, (void *) observe_string_cb, &err_info), nix::Error);
+    EXPECT_THROW(nix_err_info_msg(NULL, ctx, OBSERVE_STRING(err_info)), nix::Error);
 
     try {
         throw nix::Error("testing error");
     } catch (...) {
         nix_context_error(ctx);
     }
-    nix_err_info_msg(nix_c_context_create(), ctx, (void *) observe_string_cb, &err_info);
+    nix_err_info_msg(nix_c_context_create(), ctx, OBSERVE_STRING(err_info));
     ASSERT_STREQ("testing error", err_info.c_str());
 }
 
@@ -123,7 +119,7 @@ TEST_F(nix_api_util_context, nix_err_name)
     std::string err_name;
 
     // no error
-    EXPECT_THROW(nix_err_name(NULL, ctx, (void *) observe_string_cb, &err_name), nix::Error);
+    EXPECT_THROW(nix_err_name(NULL, ctx, OBSERVE_STRING(err_name)), nix::Error);
 
     std::string err_msg_ref;
     try {
@@ -131,7 +127,7 @@ TEST_F(nix_api_util_context, nix_err_name)
     } catch (...) {
         nix_context_error(ctx);
     }
-    nix_err_name(nix_c_context_create(), ctx, (void *) observe_string_cb, &err_name);
+    nix_err_name(nix_c_context_create(), ctx, OBSERVE_STRING(err_name));
     ASSERT_EQ(std::string(err_name), "nix::Error");
 }