考虑以下简单示例，该示例使用 ostringstream 格式化字符串和整数并丢弃输出：
#include <sstream>
void ostringstream_test() {
std::ostringstream ss;
ss << "x = " << 42;
ss.str();
}
用clang++ -S -O3 -DNDEBUG -std=c++14 test.cc编译它会生成大量的汇编代码(x86-64 指令中的汇编代码为半千字节，而类似的sprintf代码则不到一百字节( - 请参阅下面的输出。为什么它会生成如此多的代码，是 ostringstream API 固有的还是这个特定的编译器/库做错了什么？
.globl __Z18ostringstream_testv
.p2align 4, 0x90
__Z18ostringstream_testv: ## @_Z18ostringstream_testv
Lfunc_begin0:
.cfi_startproc
.cfi_personality 155, ___gxx_personality_v0
.cfi_lsda 16, Lexception0
## BB#0:
pushq %rbp
Lcfi0:
.cfi_def_cfa_offset 16
Lcfi1:
.cfi_offset %rbp, -16
movq %rsp, %rbp
Lcfi2:
.cfi_def_cfa_register %rbp
pushq %r15
pushq %r14
pushq %r13
pushq %r12
pushq %rbx
subq $328, %rsp ## imm = 0x148
Lcfi3:
.cfi_offset %rbx, -56
Lcfi4:
.cfi_offset %r12, -48
Lcfi5:
.cfi_offset %r13, -40
Lcfi6:
.cfi_offset %r14, -32
Lcfi7:
.cfi_offset %r15, -24
leaq -256(%rbp), %r14
leaq -360(%rbp), %r12
movq __ZTCNSt3__119basic_ostringstreamIcNS_11char_traitsIcEENS_9allocatorIcEEEE0_NS_13basic_ostreamIcS2_EE@GOTPCREL(%rip), %rax
leaq 24(%rax), %rcx
movq %rcx, -368(%rbp)
addq $64, %rax
movq %rax, -256(%rbp)
Ltmp0:
movq %r14, %rdi
movq %r12, %rsi
callq __ZNSt3__18ios_base4initEPv
Ltmp1:
## BB#1:
movq $0, -120(%rbp)
movl $-1, -112(%rbp)
movq __ZTVNSt3__119basic_ostringstreamIcNS_11char_traitsIcEENS_9allocatorIcEEEE@GOTPCREL(%rip), %rbx
leaq 24(%rbx), %r13
movq %r13, -368(%rbp)
addq $64, %rbx
movq %rbx, -256(%rbp)
Ltmp3:
movq %r12, %rdi
callq __ZNSt3__115basic_streambufIcNS_11char_traitsIcEEEC2Ev
Ltmp4:
## BB#2:
movq __ZTVNSt3__115basic_stringbufIcNS_11char_traitsIcEENS_9allocatorIcEEEE@GOTPCREL(%rip), %r15
addq $16, %r15
movq %r15, -360(%rbp)
movq $0, -272(%rbp)
movq $0, -280(%rbp)
movq $0, -288(%rbp)
movq $0, -296(%rbp)
movl $16, -264(%rbp)
xorps %xmm0, %xmm0
movaps %xmm0, -80(%rbp)
movq $0, -64(%rbp)
Ltmp6:
leaq -80(%rbp), %rsi
movq %r12, %rdi
callq __ZNSt3__115basic_stringbufIcNS_11char_traitsIcEENS_9allocatorIcEEE3strERKNS_12basic_stringIcS2_S4_EE
Ltmp7:
## BB#3:
testb $1, -80(%rbp)
je LBB0_5
## BB#4:
movq -64(%rbp), %rdi
callq __ZdlPv
LBB0_5:
Ltmp9:
leaq L_.str(%rip), %rsi
leaq -368(%rbp), %rdi
movl $4, %edx
callq __ZNSt3__124__put_character_sequenceIcNS_11char_traitsIcEEEERNS_13basic_ostreamIT_T0_EES7_PKS4_m
Ltmp10:
## BB#6:
Ltmp11:
movl $42, %esi
movq %rax, %rdi
callq __ZNSt3__113basic_ostreamIcNS_11char_traitsIcEEElsEi
Ltmp12:
## BB#7:
Ltmp13:
leaq -104(%rbp), %rdi
movq %r12, %rsi
callq __ZNKSt3__115basic_stringbufIcNS_11char_traitsIcEENS_9allocatorIcEEE3strEv
Ltmp14:
## BB#8:
testb $1, -104(%rbp)
je LBB0_10
## BB#9:
movq -88(%rbp), %rdi
callq __ZdlPv
LBB0_10:
movq %r13, -368(%rbp)
movq %rbx, -256(%rbp)
movq %r15, -360(%rbp)
testb $1, -296(%rbp)
je LBB0_12
## BB#11:
movq -280(%rbp), %rdi
callq __ZdlPv
LBB0_12:
movq %r12, %rdi
callq __ZNSt3__115basic_streambufIcNS_11char_traitsIcEEED2Ev
movq __ZTTNSt3__119basic_ostringstreamIcNS_11char_traitsIcEENS_9allocatorIcEEEE@GOTPCREL(%rip), %rsi
addq $8, %rsi
leaq -368(%rbp), %rdi
callq __ZNSt3__113basic_ostreamIcNS_11char_traitsIcEEED2Ev
movq %r14, %rdi
callq __ZNSt3__19basic_iosIcNS_11char_traitsIcEEED2Ev
addq $328, %rsp ## imm = 0x148
popq %rbx
popq %r12
popq %r13
popq %r14
popq %r15
popq %rbp
retq
LBB0_13:
Ltmp8:
movq %rax, -48(%rbp) ## 8-byte Spill
testb $1, -80(%rbp)
je LBB0_18
## BB#14:
movq -64(%rbp), %rdi
callq __ZdlPv
testb $1, -296(%rbp)
jne LBB0_19
jmp LBB0_20
LBB0_16:
Ltmp5:
movq %rax, -48(%rbp) ## 8-byte Spill
jmp LBB0_21
LBB0_15:
Ltmp2:
movq %rax, -48(%rbp) ## 8-byte Spill
jmp LBB0_22
LBB0_17:
Ltmp15:
movq %rax, -48(%rbp) ## 8-byte Spill
movq %r13, -368(%rbp)
movq %rbx, -256(%rbp)
movq %r15, -360(%rbp)
LBB0_18:
testb $1, -296(%rbp)
je LBB0_20
LBB0_19:
movq -280(%rbp), %rdi
callq __ZdlPv
LBB0_20:
movq %r12, %rdi
callq __ZNSt3__115basic_streambufIcNS_11char_traitsIcEEED2Ev
LBB0_21:
movq __ZTTNSt3__119basic_ostringstreamIcNS_11char_traitsIcEENS_9allocatorIcEEEE@GOTPCREL(%rip), %rsi
addq $8, %rsi
leaq -368(%rbp), %rdi
callq __ZNSt3__113basic_ostreamIcNS_11char_traitsIcEEED2Ev
LBB0_22:
movq %r14, %rdi
callq __ZNSt3__19basic_iosIcNS_11char_traitsIcEEED2Ev
movq -48(%rbp), %rdi ## 8-byte Reload
callq __Unwind_Resume
Lfunc_end0:
.cfi_endproc
.section __TEXT,__gcc_except_tab
.p2align 2
GCC_except_table0:
Lexception0:
.byte 255 ## @LPStart Encoding = omit
.byte 155 ## @TType Encoding = indirect pcrel sdata4
.asciz "303200" ## @TType base offset
.byte 3 ## Call site Encoding = udata4
.byte 65 ## Call site table length
Lset0 = Ltmp0-Lfunc_begin0 ## >> Call Site 1 <<
.long Lset0
Lset1 = Ltmp1-Ltmp0 ## Call between Ltmp0 and Ltmp1
.long Lset1
Lset2 = Ltmp2-Lfunc_begin0 ## jumps to Ltmp2
.long Lset2
.byte 0 ## On action: cleanup
Lset3 = Ltmp3-Lfunc_begin0 ## >> Call Site 2 <<
.long Lset3
Lset4 = Ltmp4-Ltmp3 ## Call between Ltmp3 and Ltmp4
.long Lset4
Lset5 = Ltmp5-Lfunc_begin0 ## jumps to Ltmp5
.long Lset5
.byte 0 ## On action: cleanup
Lset6 = Ltmp6-Lfunc_begin0 ## >> Call Site 3 <<
.long Lset6
Lset7 = Ltmp7-Ltmp6 ## Call between Ltmp6 and Ltmp7
.long Lset7
Lset8 = Ltmp8-Lfunc_begin0 ## jumps to Ltmp8
.long Lset8
.byte 0 ## On action: cleanup
Lset9 = Ltmp9-Lfunc_begin0 ## >> Call Site 4 <<
.long Lset9
Lset10 = Ltmp14-Ltmp9 ## Call between Ltmp9 and Ltmp14
.long Lset10
Lset11 = Ltmp15-Lfunc_begin0 ## jumps to Ltmp15
.long Lset11
.byte 0 ## On action: cleanup
Lset12 = Ltmp14-Lfunc_begin0 ## >> Call Site 5 <<
.long Lset12
Lset13 = Lfunc_end0-Ltmp14 ## Call between Ltmp14 and Lfunc_end0
.long Lset13
.long 0 ## has no landing pad
.byte 0 ## On action: cleanup
.p2align 2
造成差异的最可能原因是 IOStream 实现是内联扩展的，而sprintf()使用只是一个函数调用。没有什么能从本质上阻止 IOStreams 由库实现。不过，它确实需要很小的抽象和规划：标准中的定义使用模板。这些通常只是内联实现。但是，将常用的实例化(对于字符类型char和wchar_t(声明为extern模板并显式实例化它们是额外的工作。我很久以前就证明，它在编译时方面确实得到了回报，至少libstdc++在库中预实例化了IOStreams函数。根据你的实验，libc ++似乎没有。