编译单个C++文件时出现Linker command failed with exit code 1错误求助

编译一段C++代码时执行g++ test.cpp -v出现链接错误，提示Linker command failed with exit code 1，具体为arm64架构下存在未定义符号cls::cls(int)，该符号在main函数中被引用。

原代码如下：

class cls {
public:
  cls(  );
  cls(int x);
};

int x;
int* y = &x;

int main(  )
{
  // The following are unambiguously expressions, constructing instances of cls.  cls(int(x));
  cls(*static_cast<int*>(y));
}

完整编译日志：

Apple clang version 14.0.0 (clang-1400.0.29.202)
Target: arm64-apple-darwin22.3.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin
 "/Library/Developer/CommandLineTools/usr/bin/clang" -cc1 -triple arm64-apple-macosx13.0.0 -Wundef-prefix=TARGET_OS_ -Wdeprecated-objc-isa-usage -Werror=deprecated-objc-isa-usage -Werror=implicit-function-declaration -emit-obj -mrelax-all --mrelax-relocations -disable-free -clear-ast-before-backend -disable-llvm-verifier -discard-value-names -main-file-name test.cpp -mrelocation-model pic -pic-level 2 -mframe-pointer=non-leaf -fno-strict-return -fno-rounding-math -funwind-tables=2 -fobjc-msgsend-selector-stubs -target-sdk-version=13.1 -fvisibility-inlines-hidden-static-local-var -target-cpu apple-m1 -target-feature +v8.5a -target-feature +fp-armv8 -target-feature +neon -target-feature +crc -target-feature +crypto -target-feature +dotprod -target-feature +fp16fml -target-feature +ras -target-feature +lse -target-feature +rdm -target-feature +rcpc -target-feature +zcm -target-feature +zcz -target-feature +fullfp16 -target-feature +sm4 -target-feature +sha3 -target-feature +sha2 -target-feature +aes -target-abi darwinpcs -fallow-half-arguments-and-returns -debugger-tuning=lldb -target-linker-version 820.1 -v -resource-dir /Library/Developer/CommandLineTools/usr/lib/clang/14.0.0 -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk -I/usr/local/include -stdlib=libc++ -internal-isystem /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1 -internal-isystem /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/local/include -internal-isystem /Library/Developer/CommandLineTools/usr/lib/clang/14.0.0/include -internal-externc-isystem /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include -internal-externc-isystem /Library/Developer/CommandLineTools/usr/include -Wno-reorder-init-list -Wno-implicit-int-float-conversion -Wno-c99-designator -Wno-final-dtor-non-final-class -Wno-extra-semi-stmt -Wno-misleading-indentation -Wno-quoted-include-in-framework-header -Wno-implicit-fallthrough -Wno-enum-enum-conversion -Wno-enum-float-conversion -Wno-elaborated-enum-base -Wno-reserved-identifier -Wno-gnu-folding-constant -Wno-cast-function-type -Wno-bitwise-instead-of-logical -fdeprecated-macro "-fdebug-compilation-dir=/Users/sinchunyeung/Library/Mobile Documents/com~apple~CloudDocs/UST/COMP2011/practice" -ferror-limit 19 -stack-protector 1 -fstack-check -mdarwin-stkchk-strong-link -fblocks -fencode-extended-block-signature -fregister-global-dtors-with-atexit -fgnuc-version=4.2.1 -fno-cxx-modules -fcxx-exceptions -fexceptions -fmax-type-align=16 -fcommon -fcolor-diagnostics -clang-vendor-feature=+messageToSelfInClassMethodIdReturnType -clang-vendor-feature=+disableInferNewAvailabilityFromInit -clang-vendor-feature=+disableNonDependentMemberExprInCurrentInstantiation -fno-odr-hash-protocols -clang-vendor-feature=+enableAggressiveVLAFolding -clang-vendor-feature=+revert09abecef7bbf -clang-vendor-feature=+thisNoAlignAttr -clang-vendor-feature=+thisNoNullAttr -mllvm -disable-aligned-alloc-awareness=1 -D__GCC_HAVE_DWARF2_CFI_ASM=1 -o /var/folders/f3/h4301l8d7hn_1612_5g5hpmw0000gn/T/test-14473d.o -x c++ test.cpp
clang -cc1 version 14.0.0 (clang-1400.0.29.202) default target arm64-apple-darwin22.3.0
ignoring nonexistent directory "/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/local/include"
ignoring nonexistent directory "/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/Library/Frameworks"
#include "..." search starts here:
#include <...> search starts here:
 /usr/local/include
 /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1
 /Library/Developer/CommandLineTools/usr/lib/clang/14.0.0/include
 /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include
 /Library/Developer/CommandLineTools/usr/include
 /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks (framework directory)
End of search list.
 "/Library/Developer/CommandLineTools/usr/bin/ld" -demangle -lto_library /Library/Developer/CommandLineTools/usr/lib/libLTO.dylib -no_deduplicate -dynamic -arch arm64 -platform_version macos 13.0.0 13.1 -syslibroot /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk -o a.out -L/usr/local/lib /var/folders/f3/h4301l8d7hn_1612_5g5hpmw0000gn/T/test-14473d.o -lc++ -lSystem /Library/Developer/CommandLineTools/usr/lib/clang/14.0.0/lib/darwin/libclang_rt.osx.a
Undefined symbols for architecture arm64:
  "cls::cls(int)", referenced from:
      _main in test-14473d.o
ld: symbol(s) not found for architecture arm64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

错误原因 #

• 代码中仅声明了cls类的两个构造函数cls()和cls(int x)，但未提供函数定义（实现）。编译阶段因有声明可通过，但链接阶段需要构造函数的具体实现，找不到则抛出未定义符号错误。
• main函数中调用cls(*static_cast<int*>(y))会触发cls(int)构造函数，而该函数只有声明无实现，导致链接失败。

解决方法 #

给cls类的构造函数添加具体实现，有两种方式：

方式1：类内定义构造函数 #

class cls {
public:
  cls() {} // 默认构造函数实现
  cls(int x) {} // int参数构造函数实现
};

int x;
int* y = &x;

int main()
{
  cls(*static_cast<int*>(y));
}

方式2：类外定义构造函数 #

class cls {
public:
  cls();
  cls(int x);
};

// 默认构造函数实现
cls::cls() {}

// int参数构造函数实现
cls::cls(int x) {}

int x;
int* y = &x;

int main()
{
  cls(*static_cast<int*>(y));
}

编译修改后的代码，链接错误即可解决。

内容的提问来源于stack exchange，提问作者Y.T.